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I have about given up on my old B&S 12 hp vertical shaft on my Snapper RER. It started suring and stalling really bad this spring.

Here are the symptoms: it starts and runs good cold, but after about 10 minutes under load it starts bucking and surging, then finaly loses so much power that it stalls out. Takes forever to re-start and has to cool down a while first.

Here is what I have done so far:
- clean fuel tank, line, filter
- new plug properly gapped
- clean air filter
- New ignition coil
- Total carb rebuild with new throttle shaft and seals, new float, needle, and seat, gaskets, all the passages are squeaky clean. I do a lot of carbs so I know this one is set up right.
- new governor linkage and spring

I am about to buy it a new engine. This one is a 1990 build date so it's got a few hundred hours on it. But I want to try one more time and see if I missed something that would be cheaper than the re-power:

- Could this be a sticking valve that only gets bad when it's hot?
- Could it be very low compression (I have not checked compression)
- Could it be one of the safety interlocks shorting to ground when they get hot (assuming they are near the engine where they get hot)

I want to give it one more shot before I spend $$$. I can do a ring, valve, and gasket job pretty cheap if I don't need a piston or any real machinging.

Advice please!
 

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Vet-Wrencher
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253 Posts
Might check your valve clearances. They'll give you the same fit of a bad coil if there set to tight.
 

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Remove the fuel line from the carb and check the flow. If it slows to a drip, you probably have a tank vent problem, I didn't see that listed on your repairs. Valve clearance will show up as a problem on a cold engine, and will go away as the engine warms up. If you can start it cold, valve clearance is not your problem.
 

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Vet-Wrencher
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253 Posts
Not necissarily. You can have both conditions. a cold engine too and when it gets hot the valve stems can lengthen. When they do and if not set right well, there goes compression and good sealing of the combustion chamber. Can result in engine bucking or failure to run. That is, ti'll it cools off. That may or may not be your problem but if all else fails, thats were i'd go. BEEN THERE before.
 

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Looks like you covered the normal culprits already. I would suggest a check for one very uncommon problem before you scrap it, the symptoms match. In 5 years of commercial repair I encountered this problem twice in a week, it was the only time I ever encountered it. When the engine would reach operating temp the valve seat insert would slide out of the block and begin to move with the valve, with predictable results. In both cases I restaked the insert into the block and done a valve job. It is uncommon, but is worth a check before you give up on it.
 
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