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

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.