OutdoorKing Repair Forum Forums Restoring, Customising and Modifying Tech Talk Tecumseh 13.5 hp

Hi wondering if anyone can help. I have a MTD Yardman with a 13.5hp Tecumseh ( model OV358EA ) and it wont turn over properly. It winds over as if there is to much compression. I have got a brand new, charged up battery so thats not the problem. Is there a simple way to check the starter motor or regulator to see if they might by faulty. Or could it be as simple as resetting the valves ( havent played with them yet, first time i've played with a tecumseh ) if so what would the settings be. Any help or advice appriciated, Paul.

paul.foot,

Have you checked the flywheel key?

Hi Paul. The Tecumseh OHV13.5 engine is prone to a problem that is also characteristic of the large Briggs Intek engines: unless the valve clearance is adjusted accurately, the decompressor does not function and the starter is unable to turn it over. If the camshaft is worn, even if the valve lash is correct, the decompressor will not work, producing the same outcome.

Some background info would be helpful too, for diagnosis. Is this machine new to you, or is this a fault that has developed while you've had it?

BTW, valve clearances for this engine are 0.004" or 0.10mm for both valves, with cold engine, and piston at TDC on compression stroke.

I was thinking that this engine may be prone to the same decompressor issues [similar decomp design] as the B&S Intek, which grumpy has confirmed.

Just so you don't have to take my word for it Paul, look at Item 6 in the Starting Circuit Testing Procedure, from Page 43 of the Tecumseh OHV technicians' manual:



Unless the previous owner adjusted the tappet clearance regularly, there is a good chance the camshaft is OK for now, and you'll get out of trouble just by adjusting the clearance.

Hi Bruce, have checked flywheel key,all good there, a few more muscles to get flywheel nut undone wouldnt hurt though.
Hi Gadge, dont have any background info sorry, mower was half stripped when i bought it, starting with motor then on cutting deck etc.
Hi Grumpy and Gagde, have checked and reset valves, inlet was just a fraction tight, exhaust was spot on. Your coment about the B&S engine made me think of the John Deere i had the same problem with ( funny how your brain doesnt catch on till after the fact ) so I ran through same diagnosis and came up with same problem. Exhaust valve not opening slightly on compression stroke even though rocker arm is getting slightly tight but not moving valve. Is there a chance the decompression unit could be sticking or is it a case of new one needed. Will start stripping it down tomorrow and hopefully put some photos of it on for you.
Grumpy, trust me when I say I will always believe what you have to say, you have helped me out heaps and you wouldnt be doing what you do if me or anyone else thought you talked sh*t. Your like gods gift to mower mechanics everywhere.

Thanks for the compliment, Paul, but everyone gets it wrong some of the time, especially me.

It is likely that your decompressor is either stuck or worn out. I think the crankcase needs to be opened, whichever of those is the problem. Here is the description from the technical manual:


Once you get in there, it should be obvious whether the pin that bumps the tappet upward is badly worn down, or it or the pivoted weight that moves it is more or less stuck. I'm not a fan of the hairpin spring they seem to be using, but I haven't seen the inside of an OHV Tecumseh, so I'm just making blind guesses based on other brands of engine. Please post lots of nice clear pictures, Paul. This thread could become our first OHV Tecumseh classic in the archives.

Hi Grumpy, have a slight problem, have had a look around and a few phone calls for the camshaft i "might" need and one old timer told me that Tecumseh no longer exist or are making parts, if this is true could you suggest where i might be able to get one from ( if needed). Thanks Paul.

It's true that Tecumseh engine manufacture was shut down in 2008, and they no longer supply parts.
But many bits for the later model engines are still available in the US.

There are two awkward things about that situation:
1. Many US retailers won't accept international orders at all. Something else to watch out for, is will they accept non-US credit cards, or PayPal - other methods of funds transfer are exy [e.g. Western Union].
2. When you find one that will, postage can be a bit exy, even when they will use USPS [the US Post Office]. I say postage here, because couriers like FedEx, UPS, DHL are to be avoided, due to cost.

Duty/GST at the Oz end isn't a problem, so long as the order is under AU$1000, at this time.

A couple that will accept international orders [no personal experience with these; just as examples]
http://www.jackssmallengines.com

http://www.ereplacementparts.com/ - this mob ship through a third party forwarder

It was an interesting situation when Tecumseh closed, then disposed of, their engine plant and assets. Early in 2009 "certain assets" of Tecumseh Power were sold to Certified Parts Corporation, the owner of which announced a succession of ambitious plans. Here is the Wikipedia entry:
http://en.wikipedia.org/wiki/Tecumseh_Products

Here is the first announcement:
http://www.tecumsehpower.com/comet/index.html

Here is one a year or so later:
http://www.manufacturing.net/news/2010/09/certified-parts-corp-determined-to-stay-afloat

It seems the facilities to make Tecumseh spares may still exist, and there may also be substantial stocks of some spare parts.

Hi Grumpy, have got some photos so far. Have had a good look at the "pin", is slightly worn, but how much would be to much. Have measured it fully in and it sticks out about 1/32 of an inch. It still functions properly, a little bit of pressure to move the arm then it swings back with ease.

So, it isn't broken or sticking. The question is whether it is bumping the rocker arm halfway through the compression stroke. I can't see the pin in either of the pictures showing the decompressor (the last two). The second-last one seems to be the picture that comes closest to showing it, but the angle isn't right to show it sticking up past the cam lobe. Can you give us some more angles please?

If the MCR is working as it should, the pin should stick out above some part of the cam base circle, not the lobe. Poor wording, in that manual.

It can just be seen in this diagram:

Paul's last picture shows the mechanism that pushes the pin, and the end of the pin that gets pushed, but it doesn't show the end that has to stick up past the cam. We need to see what the working end of the pin looks like, with the centrifugal weight in that same position as in the last picture. We also need confirmation that the working end of the pin can't be pushed in slightly, by fairly heavy pressure, such as it receives from the valve spring via the rocker and pushrod.

Paul, here is your second-last picture, with the working end of the pin circled in red. We are looking for pictures that show us how far that working end sticks up past the surface of the cam.

Hi Grumpy have some more photos for you. Let me now if more are needed. Is there a chance it could of been sticking due to sludgy oil/ build up of gunk, as the motor was sitting for a while before i got it and started working on it. I did put new oil in it striaght away.

One very simple check you could make, is to inspect the bottom surface of the tappet which the decompressor bumps. Look for a local dent, or much more likely a groove right around, that surface, caused by the pin (tappets normally rotate while the engine is running).

If you have a sliding caliper (either vernier or electronic), please try to measure across the cam and pin, then across the cam immediately beside the pin. Alternatively you could lay a straight-edge across the cam and use the caliper to measure how far that pin sticks out above the cam.

In your pictures the mechanism looks oily but extremely clean Paul. If it was equally clean when you ran the engine, I don't think it was fouled up by gunk. Of course you should carefully feel the movement of the centrifugal weight, and of the pin itself, to verify that they operate smoothly.

The objective in all this is to get a measurement of how far the valve is being "bumped" by the decompressor during the compression stroke. If all else fails, you can reassemble the engine and watch the rocker arm for a "bump", both with zero tappet clearance and with the recommended tappet clearance. Then you can simply do a compression test with the tappet correctly adjusted, then repeat it with zero tappet clearance, and with an exaggerated tappet clearance. Because of the action of the decompressor, we would expect to find a much lower compression pressure with both zero tappet clearance and the recommended tappet clearance, compared with an exaggerated tappet clearance, which would defeat the operation of the decompressor.

Hi Grumpy, sorry forgot to do the measuring bit will get on to it asap. There is a mark on the tappet, it is a straight line about 4-5mm in lenght. Will try to get a picture. The weight and pin both move freely with no sign of sticking anywhere.Did put motor back together without base and turned motor over by hand, watched the pin closely as it went past the tappet,no movement but the tappet itself turned in a clockwise motion. Put it all back together and tried 3 different settings; closed-80psi,, .004-120psi,, .008-145psi. ( sorry bout the stuff around Grumpy, theres not enough days in the week to get done what i need.)

Hey, that's pretty good response time Paul - you couldn't get a job with most companies' service department, because you're too quick to get the job done. What used to be known as a "rate buster" in the labour movement.

I think the results are pretty conclusive, I just don't know exactly which bit is faulty. The 80 psi reading is what you'd get if the decompressor were working properly, and the 145 psi is what you'd get with no decompressor. What I don't understand is how the difference between decompressor and no decompressor is so spread-out: I'd expect a very sharp transition. I remember when my brother's Intek wore out its decompressor, it worked perfectly with the minimum specified tappet clearance, but stalled on the starter and burned everything out at 0.001" more than minimum specification. One possibility is that your decompressor mechanism may have gone a bit springy, perhaps due to wear in its pivots. You will recall I asked you to try to find out whether there is any compliance in that mechanism: whether if you push the pin inward, hard, with a small piece of metal, you can detect any movement. That would mean the mechanism wasn't pushing the pin as hard as it needs to, because of lack of rigidity.

Hi Grumpy, have tried pushing the pin in with a punch a couple of times, couldnt see or feel any movement but did end up with a sore thumb when the punch slipped off once. ( stop laughing ) Have done some measuring for you, across the cam is 1&21/64 of an inch,, across cam and pin is 1&23/64 of an inch, which means if my maths is correct the difference is 1/32 of an inch. Just scrolled through our conversation so far and stopped on the firstpicture you sent me off the decompression unit and noticed i dont have the thrust washer, it not being there wont make any difference will it? Have supplied a photo of the exhuast tappet, is the mark on it meant to be that way?

Whether 0.031" is enough depends on two things Paul: the rocker arm ratio, which for most pushrod engines is usually somewhere around 1.4, and the compliance of Tecumseh's springy-looking decompressor mechanism. Noting that you had to lift the valve itself by 0.012" less decompressor compliance to get adequate decompression, it is obvious that 1.4 times 0.031", or 0.043", is several times as much actual lift as you need for the job. However in practice you have found that the decompressor is not working. It therefore seems the decompressor mechanism's compliance is using up nearly all of your lift provided by the pin. Can you take that dimension across the camshaft and pin in two ways: with the pin being pushed down hard, and with the pivot arm and shaft on the opposite side of the cam being pushed up hard? If you are able to do that, we'll know the total compliance of the mechanism, including the slack. I'm guessing that most of it will be due to slack between the pivoted cam, and the tiny shaft it pivots on. I suspect that most of that slack will be due to wear.

With regard to the condition of the bottom of the exhaust tappet, I don't know how it got that way, but I think the decompressor pin only touches it way out near its periphery, not in there where the mark is. To see whether it is having any effect, you could check that the inlet and exhaust tappets are identical - they usually are - and try interchanging them.

The thrust washer is necessary, both to keep from wearing out the wrong parts, and to locate the camshaft axially. I'm concerned as to how it came to be missing: has that engine ever been dismantled before?

Hi Grumpy, hoping i have done this right, if not let me know and i'll try again. Photo 1 is with pin in decompression mode,measures 1&23/64 of an inch. Photo 2 would be after motor has started and pin has moved back out of the way,measures 1&5/16 of an inch. Is there any way you could find out what size the thrust washer is and exactly were it goes, in the print out it says it goes between the cover and cam, so would it sit on top of the weighted arm sitting on cam, as in my first internal photo.

Tecumseh had quite a few models of OHV engine, and I find their technical manual difficult to identify models from (I can't find any reference to a model number beginning with OV followed by a number), so I tried another approach and looked at their parts list, here:
http://www.jackssmallengines.com/Jacks-Parts-Lookup/Model-Diagram/TECUMSEH/134885/41205

That way are looking specifically at the Model OV358EA, and no other. If you slide down to the bottom of Slide 2, you see this:

This does not show a thrust washer on either end of the camshaft. It does, however show one at each end of the governor shaft - see Item 80:


I am forced to conclude that the references to the washer that I found in the tech manual, were for a different OHV engine. That would help to explain why your engine, which looks as if it hasn't been apart before, does not have a washer on the camshaft. Can you help substantiate this reasoning by showing us the end of the camshaft, on the opposite side of the timing gear from the lobes, so we can see how it is supported?

It is interesting that the US parts store shows new camshafts as available for sale, by the way.

Apparently Tecumseh changed their model numbering system in 2004. This chart from their 2005 'Quick Reference Service Information' manual has cross-references.



Yes, a Google search on the camshaft part number brings up a few US stockists.

It appears then that Paul's engine is an OHV13.5 in the manual. However I can't find any specific reference that model in the decompressor or camshaft sections. Hence the basic decompressor information, as I read it, is the ordinary MCR (Mechanical Compression Release) information, which implies that it follows Figure 38, which shows a thrust washer between the timing gear and crankcase cover. Paul's engine does not have the washer, and the parts list does not show one. Either I'm missing something, or Tecumseh missed something.

No, you had it right earlier. See the Engine Assembly section of the Tech Handbook.


Yep, the thrust washer is specific to two series of horizontal shaft OHV engines, the OHH, and OHSK [models 50-70 only for the latter]. These have the MCR bobweight on the opposite side of the camgear to the cams.

This screenshot from a parts list for one of those, shows it a lot more clearly than the Tech HB.

Thanks for sorting that out, Gadge.

Paul, your compression tests seemed to indicate that the problem is excessive compliance in your valve train. I began by suspecting that the problem was in the decompressor mechanism, but you've checked that out thoroughly, and there does not seem to be anything wrong with it. So, despite the Tecumseh manual's problem solving chart, which points strongly to the decompressor area, we need to find another source of the excess valve train compliance.

If I put the manual aside, and put experience with Intek engines aside, there are just five places excess compliance can arise:
1. Slack or worn camshaft bearings.
2. Damage to the tappet, especially its bottom surface.
3. Ill-fitting lower pushrod ball and socket.
4. Ill-fitting upper pushrod ball and socket.
5. Ill fitting or, more likely, cracked, exhaust valve rocker arm or rocker arm central pivot mounting.

1. I personally haven't seen an engine with slack camshaft bearings, but they are very easily checked, and I think this should be done.
2. You've looked at the bottom of the tappet, and there are still some mysteries to be resolved on how it became damaged, and whether that has any bearing on the compliance problem.
3 & 4. Problems in pushrod ball-joints are common on worn engines when the pushrods have been interchanged or replaced, but I have not seen them otherwise (except when there has been lubrication failure, of course, which can cause problems just as you are currently experiencing: the pushrods are usually hollow, and if the ball-joint seizes, the ball can break through into the inside of the rod below the socket. I've seen it on cars, though not on mower engines so far).
5. Rocker arm problems are not especially rare, though my experience of them cracking is on cars, which have very strong valve springs, and OHC Honda mower engines, which have design problems (especially with valve guides shifting upward so the rocker arm's movement is restricted). Problems with the central pivot mounting are of two kinds: spalling due to lubrication failure, and a multitude of problems when the mounting has been abused, for example by forcing it sideways.

So, can we have a look at the timing-gear-end camshaft bearing, and a report on whether it is noticeably slack please Paul? Then a check on the pushrod ends, and finally, a close look at the rocker arms and their pivots seems to be what we need. The rocker arms haven't been interchanged have they? Honda OHC engines are rather prone to cracking those arms - I have no experience with pushrod Tecumseh engines to indicate whether they have problems too.

Hi Grumpy, have had camshaft in both side of motor and there is a little movement but i am sure once the motor is reassembled it wont be there, have found roughly same amout of movement on other motors.
The first two photos are the top of camshaft and internal of motor were camshaft fits.

The next two photos are bottom of camshaft and bottom of sump were camshaft fits.

Now some photos of pushrods, rocker arms etc. Please note inlet peices are on left hand side of photos.

The pushrod balljoint sockets in the rockers may be looking a little ratty. I can't see them very well in the pictures, but the exhaust one might be looking worn on the side of the socket closest to the center pivot. On cars, I've seen at least one pushrod projecting right through that socket - the lubrication there is fairly poor on horizontal crankshaft, vertical cylinder engines. Also though, I seem to recall that failure occurred when someone had become unusually ambitious with the valve springs and cam contours he'd used. Most people use thick, forged aluminium rockers when they attempt that kind of thing.

How do the ball parts of the rockers' center pivots look?

In these pictures, the bottom surface of the exhaust tappet looks as if it has a black area around the outside: is that the part that the decompressor pin contacts? Can you feel any depression there?

Hi Grumpy, have had another good look and also measured pushrod lenght, balljoint ends, tappets lenght, radious ect ect.The only thing i could find that was different was the centre pivots on the rockers, the exhaust is slightly more worn then inlet, but not enough to catch your finger nail on when feeling for a lip or divot. With the tappet if you look real close (am sorry photos dont help )a coulpe of mm's inside of the dark ring you can see a slight scoring. I put the camshaft back in with tappet and turned motor over by hand while holding pressure on tappet with pushrod, and trying to get me head in at same time, that is were i judged the pin to be hitting. There is a small depression ,the dark ring, I used a metal ruler and could see a tiny bit of sunlight under each edge, but could also do the same on the inlet tappet. Going to try an experiment, going to put all timing gear back together but backwards ie. exhaust will be inlet, inlet will be exhaust. No doubt it will be a waste of time but you just never know, stranger things have happened. Will let you know how i go.

That is exactly what I was going to suggest you try next, Paul. Put in the tappet, pushrod, rocker, and rocker pivot nut, all as a set so they have run together previously and lapped themselves to fit. I suggest you watch the rocker arm bump during compression to see if it is more convincing than before, and also do a compression test with the tappet correctly adjusted. If results are the same as before, that seems to clear the valve train of involvement. If it clears up the problem, we can talk about how to go further. One option, of course, would be to just leave those parts interchanged, but it would be nice to know what was wrong, for the archives.

Incidentally the exhaust rocker's central pivot does look a bit worn in your picture.

Slightly off topic, the back of the timing gear seems to have been chewed up by the reciprocating oil pump. The pump may have been misaligned laterally and it could also be slamming the camshaft up and down across the slack in its bearing in the crankcase cover. Generally it just seems odd to me, we need to think about it.

Hi Grumpy, they reckon great minds think alike. Have done my little experimentand came up with something slightly different. I put it all back together and set valves at correct gap, turned motor over by hand a number of times and there was no opening of exhaust valve on compression stroke, but the rocker arm got tight as before. Done 3 compression checks,checking valve gap inbetween each try just incase something decided to move, and came up with 150 psi each time, were as when rockers pushrods etc were in original position i had 120 psi. Does this mean that all or some of these parts are worn out/no good or do we need to delve in deeper (to were i dont know). Could you please show me what you mean by the timing gear being chewed up by the oil pump.

Isn't that a torn up timing gear where the oil pump presses against it?

The results with the substitute set of valve train parts seem to mean it isn't the valve train. Because you had pressed hard on the decompressor pin previously and it had not moved inward to reduce the valve lift, I ended up thinking the deflection must be occurring in the valve train, but you seem to have proved it didn't. If it isn't the valve train, it seems to me that either the pin is deflecting inward (valve spring pushes it harder than you did) or the camshaft is moving in some way.

How do you feel about temporarily sticking a small piece of thin sheet metal the same thickness as the distance the pin sticks up above the cam, onto the base circle say 30 camshaft degrees ahead of the decompressor, and seeing if you get a rocker bump then? Of course you couldn't do a compression test, because the intake valve would still be open, but you should be able to see the exhaust rocker bump. If it is unclear what I'm talking about, just say so and I'll try harder to explain the idea.

Paul, in case it helps, I've measured how far the decompressor lifts the tappet on a small Honda engine (the GXV120). It is 0.054". Your much larger engine is only lifting the tappet by 0.046". It does seem as if your decompressor pin has probably worn down a bit, but I can't believe that would stop it from working.

If you can see how much your rocker bumps with a 0.046" (or thereabouts) piece of sheet metal glued to the cam, and if it is clearly quite a bit more than it bumps when the pin runs under the tappet, we'll know the pin is deflecting under load.

Hi Grumpy, sorry its taken a bit but flat out with work and will be for a few more days so please bear with me . Seems to me I have sent us in a big circle or two steps forward three steps back. Yes your right that is were the oil pump sits on the camshaft, there is a bit of scoring around were it sits but not to sure if it is to much, which i guess is were we are going. Have searched high and low in my shed for a thin peice of metal, honestly cant find anything but i cut up a pepsi can and using a few peices i have the right thickness, now got to figure on fixing it to the camshaft. Will have a look in my dads shed see if he has anything more suited to the job. If i find something what would be suitable to glue it to camshaft, would arildite do it.

I don't consider that a delay, Paul. I admit I get a bit testy when we get to this point and the other party suddenly vanishes forever without an outcome, so the whole exercise is wasted, but you are a model of cooperation and a genuine pleasure to work with.

I think either epoxy cement (araldite) or cyanoacrylate glue (superglue) should be strong enough for this job. To use either of them you will need to get the cam lobe clean, without damaging it. Acetone would be a good cleaning agent. Carburetor cleaner will degrease it well but leaves its own residue, which might be a problem. If you have isopropyl alcohol it should do the job. I think I'd give it a short blast with carb cleaner, then dry it with a clean cloth and do a second wash with methylated spirit. Remember, you only have to clean a very small area. After the test you'll need to remove the glued-on bit and oil the cam right away, or it will rust.

A one-piece bump would be better than a multi-piece one because with multi-piece you can't be sure it doesn't compress when the load is on it, due to the spaces between layers.

Hi Grumpy have done as suggested. I found an old heat shield of a B&S which was 0.047", not exactly the measurement but very close. Chamfered down all edges and glued it on last night with araldite, let it cure over night. Put it all back together today, turned motor over by hand a number of times, all that happened was as before the rocker arm tightens up but no actual valve movement, hoping i had done this right of course, or is this our way of saying that this 0.047" measurement needs to be bigger. Being that you said for a Honda it needs a lift of 0.054", then by rights we need almost double that for a larger motor wouldnt we?

It is an important result, Paul: it proves that the combination of that height of decompressor lift, and valve train compliance, can't make the decompressor work. Since the valve train isn't worn out or broken, the only way we can fix the problem is to increase the lift of the decompressor, at the camshaft.

The ideal way to wrap that point up would be if someone has access to a new Tecumseh camshaft and can measure the decompressor lift. Failing that, a measurement from a camshaft that has a working decompressor, would at least complete the record for us - but what is really needed is the correct dimension for a brand new camshaft.

It seems to me there are now two feasible ways to go. The first is to make a longer decompressor pin. The second is to replace the camshaft. Can you post some detail on how the decompressor mechanism comes to pieces, so we can talk about the first option?

The decompressor pin height does not need to be doubled, it just needs to be increased enough so that it will take a while to wear down to the point where the decompressor stops working. We know that your decompressor just barely works if the tappet clearance is reduced to zero. The correct tappet clearance is 0.004", so if you added 0.004" to the decompressor pin, it would just barely work with the correct clearance. It therefore seems the pin needs to be at least 0.008" longer if it is going to work for a few years. Making it longer than necessary is likely to impose some ugly side-loads on the pin, and might score the surface of the tappet. By far the best solution would be to get a measurement from a new camshaft, if anyone knows how we can obtain that information. If we can't find out, in the end a new pin will need to be made with a guessed length.

So:
Question 1: what pin height do we need?
Question 2: can the old pin be removed and replaced without damaging anything?
Question 3: can we get exact dimensions of the old pin, so a new one can be made (access to a lathe required, followed by flame-hardening).

Hi Grumpy, will try to get measurement from a new or near new camshaft from my end, but not likley to have that much luck. Have measured old pin by moving weighted arm back out of the way and got lenght- 0.9505" and the thickness is 0.0855". Have a coulpe of photos showing the pin that holds the weighted arm on.

As i dont have a punch thin enough no doubt i will need a press to push the pin out and new one back in. Can go down to local engineer shop and see if they are able to remove pin and once worked out to make a new decompression pin, that cant happen till monday though, lucky buggers get the weekend off.

The pin must have a head on it to keep it from coming out of its hole. Remember when you talk to the guys in the engineering shop though, an NOS camshaft is under US$80 before shipping:

http://www.ebay.com.au/itm/Tecumseh-36926A-Camshaft-MCR-OHV130-OV358EA-/190976060430

I don't know what local suppliers would be asking for one, if there is any local stock.

Hi Grumpy, have finally gotten down to engineer and bad news. They would be able to make a new pin for me but not hard enough to withstand what is needed, he did suggest another bloke who would do it but at approx $75 an hour i think it will be cheaper to buy a new one, also garunteed that way. Have had a quick look at the link you sent me about the new camshaft, am going to e-mail him as soon as i am finished here to see if he would be able to help me. will let you know how i have gotten on.

Thanks for the report, Paul. I'm not surprised that it isn't economically viable to pay someone to fix a worn out decompressor: that is why Tecumseh didn't make it repairable. (However, one of the other OHV models did have a dismantlable decompressor, of the same general design as yours. Sounds as if they may have been a little uncoordinated internally.)

Of course the upside of getting a new camshaft is that you could then tell us how long a new pin is, and this thread would then inform future Tecumseh owners on how to tell if their camshafts are too far gone, without them having to do all the work you've done. They'd just compare the lengths of their pins, with your worn out one and your new one. If they were getting close to your worn out one, it would be new camshaft time. I realise that isn't much consolation to you, having been stuck with the worst of both worlds: doing all the work, and needing a new camshaft anyway.

FWIW, I had it in mind that the new pin would be turned up out of a piece of ordinary silver steel (which I believe the Americans call "drill rod") and flame hardened with an ordinary barbeque torch - mine cost me about $5, with replacement cans of gas $1 each. I use one for my flame hardening, because I'm way too cheap to pay huge rental fees on an oxygen bottle. (I used an oxy-propane set for years, until they jacked up the rental on the oxy bottle. Now that set sits in the junk cabinet. It's a bit irritating not being able to oxy-cut, and not being able to heat up large objects, but being a cheapskate is a key part of who I am - kind of a personal value.)

Hi Grumpy, have heard back from Jason, re: e-bay link you supplied, and guess what , no luck. Have tried a couple of other places but no answer as yet. Will keep trying, cant go this far and give up. Will keep you posted , my luck has to change soon.

I'm sure the engine will end up in working order, Paul. If all else fails it can't be that hard for you to buy a pin punch to knock out that staked pin in the timing gear, and me to post you a home-made pin. It wouldn't end up being a new camshaft, but it might be fun some of the time (mostly, in hindsight years later).

Hi Grumpy, have come to the conclusion that the americans dont have this camshaft or they dont want to deal with a nobody. If i was a big company ordering thousands of units at a time they would be all over me. So, i will invest in a pin punch and if you dont mind could i ask you to make the pin for me, it will be greatly appriciated.

No problem, Paul. We just need to agree on a length for the new pin, and you need to post me the old one for measurement. I'll send you my address by PM, and you can give me yours the same way.

So, will you be happy with 0.012" longer than the old pin? If we're both still above ground when it wears out again, we can do the same repair job again, and if necessary make it longer next time.

Hi Grumpy, a couple of messages ago we worked out the pin should be at least 0.008" longer, I am thinking like you the longer it is without doing any internal damage the better it will be, as in life span. So yes 0.012" longer will do it. Will get this pin out and pass on to you.

Paul's old pin reached me today, and my first jobs were measuring and drawing it. Here is the drawing, first try:



If anyone sees anything wrong with it, please say so now, before I get started on making one. This pin is 0.012" longer than Paul's worn-down original. All of the extra length has been added to Dimension B.

Note I've shown the longitudinal dimension of the tapered section as well as the angle (35 degrees). Yes, I know the longitudinal dimension is redundant, but I thought it might make someone more comfortable some time, to have it.

Paul, I've made a new pin from what I thought was silver steel, and hardened it as much as I could, but I was surprised to find that the original pin was file-hard: somewhere near 64 Rockwell C. So, not only is the new pin not as pretty as the genuine one, it is also not nearly as hard. The genuine one was made from a fully hardenable alloy and hardened as much as possible, and I think that was because the design must have made that necessary. I'll post the old and new pins back to you: please try it and see, firstly, whether it works properly. You might also post some more pictures of the pins, the installation, and the tests. If it doesn't work, we'll have to develop a revised approach, depending on what happens.

Because it is obviously going to wear faster than the original, I made it 0.014" longer than the worn one, rather than 0.012".

Hi Grumpy, all sounds good to me, it doesnt matter what it looks like so long as it does its job. I'm suprised that the original pin lasted as long as it did being alloy, i understand it was hardened, must be some good stuff. Will charge camera up and get it ready. Been thinking, with all the photos i've been taking i might shift into photography, nope cant do that ,too many werdios.

Your photos are excellent Paul, much better than mine. The old pin is made from a steel alloy, not an aluminium alloy. Alloy just means mixture of metals, and the steel in your original pin may be an unusual one. It seems to have a ground finish, not a turned finish, and if that is correct, it would be difficult to make one at home from the correct alloy. It looks as if making one of these pins from a material similar to the original, would require doing the job with a toolpost grinder. Cutting the circlip groove would then require something like mounting a Dremel tool on the toolpost and using a specialised ultra-narrow diamond wheel, or perhaps a series of ultra-narrow grinding disks. As it happens I do not have any such apparatus. The material itself could be 18/4/1 HSS (High Speed Steel), and could be obtained without serious difficulty I think.

Anyway, step one is to see if the one I made, fits and works. If it does both those things, the thread will contain useful information for other large OHV Tecumseh owners: the diagnosis, the drawing of the part, and the finding that the throw-away camshaft can actually be refurbished.

Hi Grumpy, to me it sounds as if they (Tecumseh) are trying to show off their metal work abilitys, i understand the the stress and pressure these pins are under and what they would go through in a life time, but its showing off. Will await the pins return, and hopefully we will have a happy conclusion to this thread.

There are still potential solutions available, Paul. However, the first question is whether the home-made part I've sent works in the short term. If it does, it is "only" a materials and equipment issue to make a durable one. As you have reminded me elsewhere, I have been overlooking the possibility of using a piece of tool steel (extremely high-carbon, plain unalloyed steel) in a fully-hard, untempered condition. If there isn't much impact load, it could work.

Hi Grumpy have got some photos for you.
The old pin is on left hand side, new pin right hand side.

Assembling the decompression unit onto camshaft.




The next photo is internal of engine with all moving peices oiled and ready to go.. This is when i turned motor over by hand and checked to see if exhaust valve opened on compression stroke,,and it did beautifully.

Now i have a problem. The motor we are dicussing in another thread (millers falls )was currently sitting on this mower base untill my Tecumseh was fixed (cleaning up another base for the millers falls). Tecumseh is now fixed have returned to its home. Problem, the Tecumseh is now also hunting/surging just as the other motor is..I am wondering if there could be a wiring problem on the base itself. As i never got the Tecumseh to run until now due to decompressor and with other engine hunting/surging just as the tecumseh is, it is making me think wiring. Going to do some research try and find a wiring diagram for mower and see if i can figure it out. Will let you know how i go.

There should not be any electrical connection between the mower and the engine when you are doing diagnostics, Paul. I suggest you disconnect the kill wire at the coil and try the engine. Somewhere in the world there is a previous tenant who connects kill wires to plus 12 Volts, which plays hell with ignition modules until they suffer an early death. It is a regular source of disaster in our archives.

I wasn't thinking about the mower itself, and had not realised you were allowing it to be connected to the mower electrics. You should never do that when you are looking for a fault.

Hi Grumpy, have disconnected all wires from motor as suggested, no change. Have double checked valves, carby etc, all are set correctly and clean. I have only started the tecumseh once with the wires connected, that wouldnt be enough to start burning out the ignition module would it.
Did find a workshop manual yesterday and started looking through the wiring side, man talk about confusing, hopfully wont be needed now. Going to keep checking things, will let you know if i sort it out.

The key observation is that both engines misbehaved in the same way, when you installed them on that mower. The next step should be to put a voltmeter on the kill-wire running to the mower (with it disconnected from the engine of course) and switch the engine on and off at the engine stop switch on the mower. See if 12 Volts appears when switched on, and disappears when switched off. If it does, it may be curtains for the ignition modules on both engines. Is it possible to try both engines on your other mower? If you do that, we'll have some very clear evidence.

I don't know how long it takes to destroy the module if you put 12 Volts on the kill wire, Paul. We've had several cases where the presence of the 12 Volts was only discovered after the module failed.

Hi Grumpy, had a slight accident today. Dragged my spare base out and set up some wiring and battery. Started to bolt first motor on to run some tests next thing i knew smoke every where, over half my wires have melted, thing is i set them up the way they were when i got the base. Feel like going back to square one , tearing it all down and starting again.
Will try to get back to it tomorrow if not to busy, now wondering if i had wires set up wrong on starter selinoid, as far as i know they cant be backwards, can they?.
Might even go back to original base, let us know what you think.

I think you should start a new thread for the wiring problem, Paul. It is on a different machine, and has nothing to do with the erratic running problems you've had with two different engines when you attached them to one particular base. At this point I don't know what either of the first two mowers is (the one the Tecumseh was originally on, and the one the chonda was originally on). This thread is specifically about the Tecumseh engine. It may or may not have a damaged ignition module, following its temporary fitment to the Millers Falls mower - we have to look into that in your second thread (Millers Falls 13 hp). I suggest you name your new thread after this third mower that now has electrical difficulties, and begin it by posting details of what that mower is and what has happened to it. Next time you post to each of your two existing threads, give details of those mowers. Also, for all three mowers, make the test I asked for in my previous post: disconnect the kill wire at the ignition module, then connect the positive lead of a voltmeter to the kill wire coming from the mower, with the negative lead connected to the frame of the mower. Take a reading with the engine switched off, and with the engine switched on. Report the result for each mower in its own thread. We'll keep the three threads separate. Please don't make any more experiments without talking about it first, in the appropriate thread.

Hi Grumpy, its getting a bit confusing i know. Have done as you suggest and diconnected kill wire from ignition module. With the ignition off the tester showed 0.00, with the ignition on the tester shower 0.44. As far as i can figure out i shouldnt have power going to that wire but coming that wire once motor has started, is that right.
To answer a quick question, this Tecumseh motor was originally on this base MTD Yardmachine, the one it is on again.

As I said in the Millers Falls thread Paul, we just don't know what is happening yet, we need some data. The first thing we need, is your report on what you see when you put an in-line ignition tester on both engines, with the kill wires disconnected. You will have to stop the engines by pulling the ignition leads off the spark plugs, since you won't have a kill switch during those tests.

Hi Grumpy, have done the test, built an in-line tester as i said i would, not pretty but works. Made the tester so i can open the spark set from approx 1mm to 20mm, after that the spark has to jump to far and motor will stop. While doing the test i had a constant strong spark, there was no missing at all. Did not stop it from surging/hunting though, but as with the Millers Falls motor was able to close choke up a bit of the way and smooth it out.I guess this means there is nothing wrong with the ignition module.

Yes, it sounds as if the ignition module is in the clear. However you should still have the engine isolated from the mower's electrical system when testing it: the more irrelevant variables you can eliminate, the more likely we are to find the actual problem.

Have you had the Tecumseh hunting previously? We started out dealing with a decompressor problem: did it also have a hunting problem all along, or has that just begun since you got it running again?

Hi Grumpy, when i first got the mower it wouldnt turn over properly due to decompressor problems, so never had it running until we fixed that problem. Did contact previous owner,regarding the way it used to run, apparently it hasnt ran for over 1 year and being an older gentlemen ( no offence to him ) he cant remember exactly how it used to run. All he can remeber was his sons abusing it and him fixing it until he said no more, and thats were its been sitting since.
Would you like me to get some photos of carby, complete and broken down, governor linkages etc, or are we going to go in a different direction.

If it is hunting, it either has friction in the governor linkage, or lean mixture. Lean mixture would probably be either an intake air leak, or a carburetor fault. I suggest you begin by checking that the linkage moves freely and doesn't rub against anything, and post some pictures of it. I'd like to check that it doesn't have a stretched spring or other damaged parts, since it has reportedly been abused systematically by kids.

Once we are happy with the external governor bits, the next step would be to check the fuel flow to the carburetor, by checking whether it runs freely from the pipe when you disconnect it at the carburetor end. If there is no problem in that area, it will be time to remove the carburetor for examination and cleaning - which also gives you a chance to look at the insulator gaskets, and check that the clamped joints are clean and properly clamped. If there are no suspicious items in all that, next step will be to dismantle and clean the carburetor. The idea is to avoid dismantling things that might not need to be dismantled - start with the easiest things first.

Hi Grumpy, have some photos for you, sorry they arent the best, trying to get behind the throttle mechanism is a pain.




I know the governor arm looks a bit beat up but it is not rubbing on anything and it only takes a bit of pressure to open and close governor, feels about the same as other bigger motors i have worked on. Curious question, should there also be one of those long skinny springs around the governor arm as well, or would that one big spring be all it needs.
Have gone ahead and checked fuel flow rate and it is good, runs at a nice steady flow. ( Not trying to jump ahead, just done it while taking photos ).

Let's start from the manual if we can. Here is an illustration of the OHV13 (your engine I think) governor linkage, correctly set up:


Does yours look like that? I'm wondering if various bits are attached in exactly the right places, especially the governor spring.

Hi Grumpy, looks to be the same set up. In the picture the governor spring is attached on the bottom of governor arm, hope you dont mind but i gave it a try there, no difference. Tried it up higher,just in case, made no difference. Have got some photos of carby with gaskets still on it.
This first one is the airfilter side. Is original gasket and still looks good.


Next photo is back end of carby which bolts onto inlet manifold. Gasket is homemade i'm guessing, but still looks good without no signs of wear and tear, but should it continue all the way into mouth part of carby ( around bottom left side ). With it this way would that be enough to make the motor hunt/surge.


Have included a few photos of carby itself just so as you know what we are working on.

Is there an insulator, with another gasket on the other side of it, between the carburetor flange and the intake port?

The carburetor flange gasket looks rather woeful, Paul. If it is of uneven thickness, it will leak air. It can be difficult to tell without running fingers over both sides. I think I'd make a better one, partly because it might leak, and partly because it is hardly something to be personally satisfied with.

Here is the workshop manual's advice on hunting:


It sounds rather defensive to me: I suspect they think their governor tends to be a bit unstable. However without having actually heard your engine, I think lean mixture is a more likely issue than a governor fault.

Your carburetor is a Series 7, as shown here:


Please check the illustration and description against your carburetor to confirm that identification. It seems we have to face up to the unlovely task of cleaning a Tecumseh carburetor.

Hi Grumpy, no there isnt an insulator and gasket on intake side of carby just the one gasket between the carby and intake. I have noticed there are no air ports as on other carbys either. And yes i will most defiantly be changing that gasket, i'd like to think i could do better.
The picture of carby is the same as mine, they seem a bit of a weird idea, but each to their own. Will strip carby down asap, with photos to go with it.

that is very similar to a tecumseh vector carb, does it have O rings sealing the bowl to the carb body?

Paul, cleaning a Tecumseh carburetor is fraught with difficulties. Please look at these instruction pages first:




Remember, you have a Series 7 carburetor, which is a slightly odd one. The objective is to clean your carburetor, in particular the main jet and the "main nozzle emulsion" (that is, the emulsifier). Don't dismantle more than necessary, and don't adjust anything without discussing it first. Tecumseh's quaint approach to carburetion is probably the main cause of their engines being scrapped as unfixable.

We had a fairly long thread recently on an OHV Tecumseh carburetor:
https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=55609&page=2
The main problem in that case was that the emulsifier was pressed in and could not be removed. However your Series 7 has a removable emulsifier. Let's go at this carefully, though.

Joe, this is one of the large engines, specifically an OHV13 with a Series 7 carburetor. It has an O ring on the bottom of the emulsifier, and another one on the end of the main jet.

Hi Grumpy, thanks for the info, was able to print it out and take out the shed.
Have stripped down pretty much the whole carby, given it a blow out and followed up with some carby cleaner





Before i put it back together whats your opion on this gasket, it sits between the carby body and bowl.Looks as if it had been sand blasted doesnt it. Think i had better change it.

Very nice pictures as usual, Paul. That makes it much easier to see what is going on.

The float bowl gasket looks OK, it has a design that doesn't call for it to have a smooth surface as long as it is slightly soft. The top of the float bowl appears to have a raised up ridge all the way around it, and this ridge presses into the deformable gasket material. You can see the impression it has made in the gasket.

How did the main jet and emulsifier look before you cleaned them? Unless they were slightly gummed up, the cleaning you've done won't make any difference to how well the engine runs.

Hi Grumpy, thanks for the compliment.

Good to hear the gasket is good, it is soft and slightly flexible. I was thinking that with all the pits on the top side it might of been able to let air leak in around it. Thats sorted now. Your not going to believe me but the inside of the carby was virtually spotless, it looked as if was had been cleaned recently. Should i put it back togehter and give it go now or should i go deeper.
Curious question, the way it sounds to be running could i have put the motor back together maybe 1-2 teeth out. I am 99% sure i done it right, piston at TDC and all timing marks lined up just as it was when i took it apart, just starting to second guess myself. I supose if it comes to it i'll have to pull it down to check, not yet though.

It is difficult to be confident about what is happening without actually seeing and hearing the engine, Paul. You will recall that we investigated your Millers Falls chonda problem as if it were hunting, when it actually was ignition cutting in and out. They sound different, but until you've practised for quite a while, you would describe them both the same way. That is why I am not really confident that your problem with this Tecumseh is lean mixture. I do not think it could be a retarded camshaft: I'm regrettably familiar with the symptoms of that, from driving old cars with worn out timing chains, which occasionally "jump" their timing. It makes the engine very gutless rather than causing any other drivability problem (though of course it overheats the exhaust valves and destroys them if you try to drive it in that condition). An advanced camshaft, on the other hand, is mainly characterised by lousy idle quality and poor off-idle acceleration, rather like the behaviour of a racing camshaft. We need to be satisfied with your carburetor before pursuing camshaft timing as a possibility.

You have dealt with the main jet and emulsifier, which are the most likely causes of lean mixture in the upper and medium speed ranges. However while you are working on the carburetor, I think it might be as well to clean the main air bleed, just by squirting carburetor cleaner through it. Look through this instruction set on the Series 7 carburetor:
http://outdoorpowerinfo.com/repairs/tecumseh_series7_carb.asp

Go to Item 14 for details of the air bleed. Do not remove the choke plate from its shaft, just see if you can blast the cleaner into the hole outlined in pink in that picture, and verify that clean carb cleaner comes out from the vent, outlined in blue. If crud comes out from the vent, it was worth doing. If perfectly clean carb cleaner comes out, it was a waste of time except that now you know it is clear. Then put the carburetor back together carefully and give it a test run.

Hi Grumpy, having a bit of trouble understanding whats happening. Will try to explain it with pictures from link you provided.

When i spray carby cleaner into the main air bleed ( pink hole ) it comes out the bottom of carby body via the " white hole ". (picture two )
When i srpay it into the bowl vent ( blue circle ) it come out the bottom of carby via green holes. ( picture 2 )

That is no problem, Paul. I was actually taking a guess/making an assumption. If the cleaner comes out freely and clean, it is all good.

Because you reported that the hunting happens at high speed, and the idle is fine (at least that was my interpretation), there is no point in assaulting the idle system with carb cleaner. As long as you are confident the main jet and emulsifier are really clean, it is time to put it back together and try it out.

The main jet and emulsifier on that carburetor are both made from plastic, so the use of carb cleaner on them should be kept to a minimum. However I have never been able to get hard gum out without using either cleaner or a jet drill. Since we don't know the sizes of the holes, we seem to be limited to carb cleaner in this case. I suggest you measure the main jet size, and the size of the tiny holes in the emulsifier, with jet drills, and report the sizes here. Put the drills in blunt-end first of course, so you don't make the holes bigger - it is only plastic you are dealing with.

Hi Grumpy, sorry it been awhile been flat out, sheds been full of generators.
Got some measuremnets for you, have had to use my missus sewing needles again, i dont have drills small enough ( i know i need some, funds are a different story ). Did do it carefully, not trying to shove an oversized needle through.
Main jet came to 0.90mm.
Top hole on emuslifier is 0.55mm,, the bottom two holes are 0.80mm.

Thanks Paul, those numbers will give people a reference when they grapple with their own Series 7 carburetors. Now all we need is for you to demonstrate that the hunting problem has gone away: otherwise, we don't know for sure that those sizes give good results.

Hi Grumpy, gave the motor a run but had no difference. Done some research and found a number of cases same as mine, most of the answers pointed to the governor not being set correctly. Hope you dont mind but had a play with governor restting it in different positions, did have it running without hunting but it was reving way to hard even in fast mode. Tried adjusting it a fraction more , back to hunting, other times it sounded as if it was getting to much fuel, when i took airfilter case off it was wet and spark plug as well. Have tried to find the corrct setting for governor but unable as yet,, wondering if you could help out please. Will keep looking myself. Let you know if i can find it.
Have included photo of governor arm, it is a two peice. I am wondering if one peice should be set one way, the other peice set another.

Is the linkage the same as in the manual's illustration, Paul?



I'm asking because I can't see the choke linkage in your picture, but it is shown in the illustration. You now have the spring in the correct notch according to the diagram.

You have reported that the governor is cycling, but with some new and very important information: the spark plug is wet, and fuel has been spraying back through the carburetor onto the air filter. I think this makes it unlikely that the problem is lean mixture. My new guess as to what is wrong, is based on that piece of information. Let's go back to the Tecumseh diagnostic procedure for governor-cycling:


When I read that the first time, I thought it was a clear admission that the performance of that governor is marginal at best, and it is prone to cycling. Now that you have said that the plug and air filter are wet with fuel, a possible explanation comes to mind. When the throttle opening increases on a carburetor, a design feature called "acceleration enrichment" comes into play. If the throttle actually flutters, the mixture is likely to become very rich. So, what I suggest you do, is follow Item 1 in the diagnostic: hold wire link between the throttle and governor arm perfectly stationary with a pair of pliers, and see if the engine then runs steadily, and changes speed when you move the throttle slightly. If it does, you will have proved that the problem is governor instability. Of course we then have to try to figure out the cause of the instability, but at least we'll have a focus.

Hi Grumpy, sorry about the time delay,, after x-mas rush is on.

The reason you are not able to see the choke linkage in my photo is i removed the throttle set up so i was able to get a good clear shot of the governor arm for you, and yes my set up is excatly like in the diagram you supplied.
Have done as suggested and held the governor wire with pliers and gave the motor a run.. It will only do this in idle mode. I am not able to open up the throttle to far as it will conk out straight away. Going by the governor arm on the carby it is running in idle mode but to me, just listening to how it runs, it seems to be running to fast as if on half throttle. Would it worth trying the governor in a few different set ups, might snag a winner.

I don't really understand what you've just told me, Paul. Why not try removing the wire link from the throttle butterfly, and controlling the engine with fingers on the butterfly lever? Don't let go of it of course, or it will probably blow the engine. As an alternative, you could rig a piece of stiff wire from the throttle butterfly to a stationary part of the engine (not part of the governor) and try to vary the engine speed by bending the center of that link. The idea is just to see whether the engine runs properly when the whole governor system is eliminated. If it won't run properly with the governor excluded from having any effect, we have an engine problem of a fairly unusual type, and I think you are going to have to go back into the crankcase to re-check the camshaft timing and inspect the decompressor mechanism.

Just to ensure you are as uncertain as I am, I'm pondering what the symptoms would be if the decompressor is stuck in the start position. Some years ago I worked on a couple of veteran cars which had manually-controlled decompressors because at 4.6 litres and with a high compression ratio (they were factory-team racing cars), there was no way you could crank them without decompressors. I even verified that by using a starter motor to rotate the crankhandle without decompressing: it immediately sheared the cross-pin in the front of the crankshaft, where the crankhandle engages. The Hulk himself could not have cranked those engines. So, the point in this story is, when you started the engine in its decompressed state, you had to move quickly to disable the decompressor, or the engine would stop. Meanwhile, it ran absolutely atrociously.

So, if your decompressor is stuck ...

Hi Grumpy, have done as suggested, removed the wire governor arm and controlled the carby myself. Did not make any difference, still not able to increase revs past idling.. I guess now i have to tear the motor back down, something we were hoping not to have to do. Will get the camera ready.

I agree Paul, it is time for that. I can't guess whether it will be that the decompressor is stuck on, or the camshaft timing is a couple of teeth out. If it isn't either of those, I'll be waiting for a useful clue from what you do find out.

I just thought I'd throw this in, it's the butterfly stuck in the manifold as is when I pulled it apart. The engine (12.5 briggs) would still run and I don't know how but only at idle and have never come across it again since. I found the two butterfly screws in the muffler . Lucky it didn't pop a valve seat or even worse wreck the engine .

Perhaps that carburetor had been serviced the way we are told to do it in the carburetor manuals, Rob: remove the choke and throttle butterfly plates, and pull out the shafts they mount on. I literally never do it unless the screws are already loose. As a lad many, many years ago I took some car carburetors apart, and was never happy with the security of those screws afterward. Because the shaft is normally brass, I ended up soldering the screws in, getting them hot enough so that the solder penetrated the threaded section. None of them ever came out after that, but I stopped taking the screws out anyway.

I've never had an engine eat a butterfly valve plate screw, but once again way back, I drove an old 800 cc OHV Morris Minor. One night I was putting along through the suburbs when suddenly the engine rattled and lost a cylinder. I think I had the ignition off in considerably less than a second, and caught a tram home to pick up the spare car and drive back, with a rope and a co-driver, to tow it home. Next morning I pulled off the head and found I'd been very lucky. A disused weld-nut had fallen off the inside of the giant oil-bath air cleaner, and slid down the intake manifold straight into the cylinder. The loose nut in the cylinder had immediately hammered the spark plug gap closed, and hadn't done any other damage in the brief time it had been running. So, it was going again in about half an hour at no cost. It could have been a great deal worse with another few seconds of running: a broken piston crown at least.

Hi Roebuck , thanks for that, but it isnt my problem, as much as i wish it was. Have had carby apart and it all works well. Have had similar problems on smaller B&S motors, but not on a bigger motor yet, touch wood. Thanks for the input Roebuck.
Hi Grumpy, have broken down motor and got some photos for you. As when i first broke it down i set it on TDC, dont ask why i just do, and as you can see from photos all timing marks are lined up.
First photo is crankshaft and counter weight.

Second photo is crankshaft and camshaft.

Have also got a few photos of decompression unit which is still in good working order,(thanks Grumpy),pin moves freely with no sign of sticking.


Did find something though. After doing what i have done so far i noticed some oil on the underneath of the head, which i have not noticed before. Got some photos so you can see what i am thinking.


I am wondering if while the motor is only idling everything is good but when i try to increase the revs some of the compression is actually be pushed out past head gasket which could be causing the burn mark and oil build up,, and lack of power . ( yellow circles ) Correct me if i'm wrong.

The mark is suspicious Paul, but not definitive by itself. It looks as if the gasket has delaminated somewhat when you took the head off the first time, and that does usually cause leakage when you put it back together. You should always scrape the surfaces of head and block so they are smooth and shiny, then inspect the gasket and if one of its surfaces is damaged, you need to replace it. I don't think the gasket was blown when you got the engine, but it may be now. You did take the head off at the beginning of the project, didn't you? That could be where the problem came from.

The first think to do is look closely at the surfaces of the gasket that correspond with the mark, and see if they show carbon, or are eroded by gas flow. Look carefully around the carbon ridge around the cylinder bore side of the gasket, and see if it is missing in places where there might have been leakage. Then see if you can get the layer of delaminated gasket material to flake off the surfaces of both head and block, without actually scraping it, by sliding a sharp blade underneath it on both sides of the suspected blow-mark. If you succeed, see if the blow mark runs across the surface where the gasket was. If you find a mark across the gasket area on either head or block, it was blown. Similarly, if you find carbon deposits on the split gasket surfaces, it was blown.

Hi Grumpy, have gotten abit of the old head gasket off the block where the "blow out " is and yes there was a blow mark across it as you mentioned there would be. ( sorry forgot to get a photo ). Have cleaned up both sides and got a new head gasket.
I will cut this next bit short,, while doing this this morning i had an old timer turn up to question me about his generator, asked what i was doing, explained the situation from the start and he proceeded to tell me to put it back together give it a run while spraying a light mist of fuel into carby throat , at same time up the revs, if it goes hard that will tell you if your carby is playing up or not. Even when i told him i have checked carby a couple of time, he stubbonly told me to try it. Walked away mumbling something about young blokes not wanting to listen...
What do you think, should i give it a go?
I am leaning towards giving it a go as i cannot think of anything else it could be.

Right now the best "it could be ..." is the head gasket, Paul.

So far as spraying fuel into the carburetor is concerned, I don't see it as having much different effect from partly closing the choke, which we already know, made it run better. However it also made it wet its spark plug and plaster its air filter with puffed-back fuel. As a general rule, don't spray fuel into the intake of an engine that is puffing back through the carburetor, for safety reasons. A faceful of burning petrol is definitely not something to look forward to.

As an aside, when you are opening the throttle is one of the times engines are most likely to puff burning fuel back through the carburetor. It is called a "spit", and is a symptom of lean mixture. The old guy may or may not know what he is talking about in general, but in this case, simple prudence should tell you not to do it.

At the moment the engine will not run at steady speed, so doing tests while opening the throttle simultaneously is rather irrelevant. It mixes two variables: steady state running conditions, and acceleration conditions. His test would be relevant (though excessively dangerous) if the engine would run at steady speed, but would not tolerate an opening throttle situation. Poor running then would indicate insufficient acceleration enrichment. By the way, the classic sign of mildly inadequate acceleration enrichment is the engine hesitating. Severely inadequate acceleration enrichment is often indicated by a spit. Spits are a bit disconcerting when the air cleaner is in place, but with it removed, they are rather frightening. No matter how well an engine is running, never stand in front of the air intake with the air cleaner removed. Have you ever seen anyone who's had a faceful of burning petrol, even several decades before? It's a sight you won't forget.

Hi Grumpy,, dont stress wasnt really going to do it, just feels as though we arent getting anywhere. I know we have in reality, just this not wanting to rev out is bit fustrating.

Have put motor back together, setting it up the same way as when tearing it down. Fresh oil, couple of turn overs without sparkplug lead on, just making sure to have everything lubricated. Gave it a run hasnt made any difference.
When i explained to you that the air filter casing was getting wet from fuel, could this have something to do with the inadequate acceleration enrichment that you are talking about. How would we go about fixing this problem, or do you think it could be something else.

You fixed the blown head gasket and it made no difference?

At this point I'm concerned about not actually hearing and seeing this engine, Paul. Let's focus on this for a minute: how well can you get it to run, by applying some choke? Does it run "better" or "properly"? Choose one.

Hi Grumpy, with the motor idling there is a very very slight surge, nothing to be worried about. When i try to rev it out to at least half throttle i need to apply approx 1/4 choke it will run properly, but at the same time it seems to be running/reving to hard, almost as if it is on full throttle plus a bit more. Hope this is understandable, if not i will try again.

What you have said seems clear Paul, but what is causing it, less so.

You said several posts ago, that when you advance the speed control without applying choke, not only does it run badly, but it develops a wet spark plug and a wet air filter. Can you check if the plug and air filter are wet after you run it on the right amount of choke, then verify that it does those things consistently (in other words, run the test again with no choke, and with choke, and see if it always shows wet plug and filter with no choke, but not with the right amount of choke to make it run properly).

Hi Grumpy, have done as suggested, hope you are ready for this.
Gave the motor a run with airfilter on, missing slightly on idle, but now i am able to rev it up to full throttle. Turned engine of , checked for any fuel around airfilter and spark plug, all was dry. Done this 4 times just to be sure it wasnt playing tricks on me, same outcome each time!!
Cannot understand why it went from not wanting to rev out to now reving out when i havent changed anything or done anything different. It is making me come back to a problem with the governor again, not so much in the way it is working but in the way the governor arm should be set up. Am wondering if someone else has tried to adjust it originally and i just havent found the correct setting for it.
Do you want me to also try running it with the choke partially on or would this now be not needed.
Talk about going around in circles.Maybe this is why some people stay away from Tecumseh motors, you never know what they are going to do one day to the next.