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?