7.5hp Sears will run out of water but not in.

[proaudio12]

I just acquired a Sears 7.5hp from a friend to see if I can get it working. The engine will run out of water but when placed in water it will stall and not start but will start again when out of water. Does this sound like a piston rings issue and not having enough compression to force the exhaust out? It was last used about 8 years ago and stalled during use and only runs out of the water now.

Also does anyone know what the compression in psi should be, I have a compression tester but have not checked it yet. Thanks for your help.

Chris

 

[whywhyzed]

Re: 7.5hp Sears will run out of water but not in.

There's a couple of different sorts of 7.5Sears. There's an air cooled Tecumseh thing, and then there's a Mcculloch built type - any numbers or ID will help, but it sounds like you are on the right track.
The mccullochs run around 110 compression IIRC

 

[steelespike]

Re: 7.5hp Sears will run out of water but not in.

Check the compression.Ideal minimum is 100 psi. and within 10% between cylinders. It will run with less down to about 60 psi with the same 10% rule.
Could also be a fuel supply problem.

 

[proaudio12]

Re: 7.5hp Sears will run out of water but not in.

It is a 7.5 hp Ted Williams, model# 217-5888-1, Tecumseh engine model # 640-06B. I am getting PSI readings about 55psi when I give it a few good pulls. Let me know what other good info you need.

Chris

 

[CATransplant]

Re: 7.5hp Sears will run out of water but not in.

That's way low on the compression, and would explain why it won't run in the water. Once the engine is in the water, you have quite a bit of back pressure on the exhaust. The little relief holes halfway up on the leg just won't let it run with that low compression.

So, now, the question is why the compression's that low. I'd pop the head off and inspect the cylinder walls. If they're scored, then your engine is toast. Those are not worth trying to fix, frankly.

If there's no scoring, then inspect the head, cylinder, and head gasket. If there are signs of a leak, check the head for warping with a straightedge in all directions. You can dress the head down on a piece of glass with emery paper. Just work in several directions to keep it flat. A new head gasket, and you should be home free.

It's just possible that you have carbon making the rings stick. If the head has a heavy coating of carbon, you might suspect that and try to decarbonize the engine with seafoam or another similar product.

But, remember, if the cylinder is scored, part the thing out on ebay.

 

[proaudio12]

Re: 7.5hp Sears will run out of water but not in.

I popped the head off, it was a little challenging with screws instead of bolts. There is a little carbon on the piston and head but the head seems to be flat with no signs of any leaks. The cylinder feels completely smooth as I run my finger around it. It is possible to get rings for this engine? I will try to clean up the head, re assemble and see what happens.

 

[steelespike]

Re: 7.5hp Sears will run out of water but not in.

Clean up the head and install a new gasket.Give it a good decarb treatment it may loosen up the rings.
These motors are marginal performers at best. In my opinion not worth a huge investmemnt in time or parts.It does have a water pump to cool the leg.
It will run without but very hot and could be why the low compression now.