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

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.