Edger with 3.5 HP Tecumseh - Exhaust Vavle Leakage

ZeeOSix

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My neighbor gave me his old Troy-Bilt gas edger with a 3.5 HP Tecumseh engine because it barely ran. He bought it used many years ago. I found out it was made in 2006. So I played around with the carburetor and could get it to run a bit better, but nowhere adequate enough to edge efficiency without it dying every 15 seconds. It's pretty old but in decent shape except for the running and the blade belt looks cracked in a few places (new one is only $5 on Amazon). Anyway, I ended up buying an aftermarket carburetor for it on Amazon for $14.50 after finding the Tecumseh carb part number (640339). It also even came with the gasket that goes between the carb and intake manifold. Slapped it on today and the edger runs pretty good now.

Anyway ... my real reason for starting this thread is about a cylinder "leak down" and compression tests I did before deciding to buy the carburetor. Did a compression test cold and dry (no oil squirted into the cylinder) and it came out to 83 PSI. I didn't squirt any oil in the cylinder to do a "wet" compression test, nor a compression test when hot. For a "leak down" test I used my air compressor with a blow nozzle with a rubber tip on it to pressurize the cylinder through the spark plug hole. I slowly turned the motor by hand while blowing air into the cylinder, and listened for escaping air through the intake valve (at the carb) and exhaust valve leakage (at the muffler).

I concluded the intake valve shows no leakage when it shouldn't leak. But I could hear slight air leakage past the exhaust valve when it shouldn't - specifically on the compression and power stroke when an exhaust valve should normally be closed. I also found and download the Tecumseh engine manual and they talk about some engines using "compression reduction" methods to help starting. I found the parts diagram and camshaft part number and found photos of the cam and can't tell if the exhaust lobe is designed to reduce compression by cracking the exhaust valve. I could however tell from the camshaft photo that it does not have the centripetal mechanism to reduce compression for starting. Is it possible they designed the camshaft to always keep the exhaust valve cracked open on the compression and power stroke? I doubt it, but who knows for sure. The engine doesn't pop or sound weird when running, so the valve leakage doesn't seem to effect how it runs. And the "cold dry" compression of 83 PSI sounds about right for an engine like this.

Anyone have any experience with small Tecumseh engines to know if it's normal by design if the exhaust valve is cracked open or is it just too tight? The engine is Tecumseh model LV148EA, 3.5 HP (148cc). Vertical crankshaft and horizontal rod/piston.
 
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check the valve clearance on the rocker arms first. This is pretty common with small engines especially that old. The vibration loosens or tightens the nut on the rocker arm over time.

If your intentional leaky valve theory is correct, run lower octane gasoline only (87 max). lower octane burns better in lower compression engines.
make sure and fuel stabilizers or additives don't increase the octane. Maybe look at the plug heat range and try one step hotter. Replacing with a good NGK plug has shown to make a difference as well vs some of the off brand plugs that come with small engines.

Sounds like you are on the right track.
 
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The hiss is an exhaust valve that is warped or burned which would make the engine hard to start and have reduced torque, especially at low speeds. I bought a brand new Tecumseh once upon a time that came from the factory with a leaking exhaust valve so it happens.

The compression relief via intentional valve leakage only cracks one of the valves open a few thousandth's of an inch for about the first half of the compression stroke so everything should be sealed tight during the last half of the compression stroke and power stroke.
 
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If it runs, has good power, starts easily, I'd leave it alone. If it bothers you, you can pull off the head and make sure there isn't carbon built up on the seat of the valve. I have cleaned the seat and valve with a wire brush.The valve shouldn't be able to be turned when fully closed on its seat.. Valve clearance will actually get tighter as the valve seat wears. As a last resort if the valve isn't warped, you can try relapping the valve and rechecking the clearance.
 
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If it was me...
just run it.. if it blows you can either junk it or repower depending on how handy you are.
 
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You can't run a compression test on a small 4-cycle engine. These small engines have a compression release, if they didn't you wouldn't be able to pull them over by hand. If the engine doesn't have enough compression to run, it simply wouldn't run.
 

ZeeOSix

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check the valve clearance on the rocker arms first. This is pretty common with small engines especially that old. The vibration loosens or tightens the nut on the rocker arm over time.
This engine doesn't have rocker arms or valve clearance adjusters. Only way to adjust the valve clearance if too tight it to grid off some material on the end of the valve stem.
 

ZeeOSix

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The hiss is an exhaust valve that is warped or burned which would make the engine hard to start and have reduced torque, especially at low speeds. I bought a brand new Tecumseh once upon a time that came from the factory with a leaking exhaust valve so it happens.

The compression relief via intentional valve leakage only cracks one of the valves open a few thousandth's of an inch for about the first half of the compression stroke so everything should be sealed tight during the last half of the compression stroke and power stroke.
Yes, I agree that it seems strange that they would design the exhaust valve to leak the whole time over the compression and power stroke. My cylinder leakage test sounded like the leakage as the same volume through out those piston strokes.

It's probably carbon or some kind of damage on the exhaust valve seat or maybe the valve clearance has tighten up over it's life for some reason. I'm probably just going to run it like it is since it still seems to run pretty good with the new carb now. Power seems good enough to get the job done.

I don't like how insensitive the governor is however to engine RPM/load. The engine needs to almost die with low RPM before the governor kicks in and opens up the carb butterfly. Don't know if the governor is adjustable to be more sensitive to the engine RPM decreasing from a load on the engine. This is a strange setup, because there is not throttle/speed control on the carb except for the "idle screw" which you adjust to set the speed you want the engine to run at all the time. Must be an edger thing.
 

ZeeOSix

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If it runs, has good power, starts easily, I'd leave it alone. If it bothers you, you can pull off the head and make sure there isn't carbon built up on the seat of the valve. I have cleaned the seat and valve with a wire brush.The valve shouldn't be able to be turned when fully closed on its seat.. Valve clearance will actually get tighter as the valve seat wears. As a last resort if the valve isn't warped, you can try relapping the valve and rechecking the clearance.
I agree ... not worth messing with at this point. I got the edger for free and have a total of $30 (including tax) for a carb, NGK spark plug, air filter and blade drive belt. A new edger like this one cost $300+. It runs good enough for the minimal usage it will get, and it doesn't smoke or seem to burn oil.

It probably is from valve seat wear. I'd have to remove the head, remove the valve and grind some metal off the end of the valve stem to make more clearance between the valve and tappet that runs off the camshaft.
 

ZeeOSix

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You can't run a compression test on a small 4-cycle engine. These small engines have a compression release, if they didn't you wouldn't be able to pull them over by hand. If the engine doesn't have enough compression to run, it simply wouldn't run.
The compression test came in at 83 PSI (cold engine, no oil squirted into cylinder), even with the leaky valve so that's why it still runs pretty good. After doing the cylinder leakage test and seeing a photo of the camshaft assembly for this engine, I don't think it has a compression release unless they purposely designed it to have the exhaust valve cracked open the whole time through the compression and power strokes ... which seems like a strange way to do it.
 

ZeeOSix

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Here's the section out of the Tecumseh Shop Manual that discusses the compression release mechanisms. I know it doesn't have the mechanism shown in Fig. 38 because I found photos of the camshaft P/N 37575 called out in the parts catalog for this engine. Photos of the camshaft is show below too, and it doesn't look like the exhaust lobe has any "bump" on it either. If it has the "Ramp Compression Release" (RCR) then it certainly can't be seen and it would still surprise me if the RCR kept the exhaust valve cracked the whole time ... but maybe so - ?.

1631482551348.png


Note - the exhaust valve lobe is actually the one on the end (the wider lobe), not the lobe near the drive gear like shown if Fig. 39 above. I verified that by looking at the parts catalog for this engine, which is also shown below.

1631482878981.jpeg


1631482977567.jpeg


1631483593993.jpeg


Exhaust valve is #125, which would be actuated by the larger cam lobe fartherst away from the drive gear. Cam is #50. Tappets are #48. Muffler is #275.

1631483151067.png
 
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