Poor compression-4 stroke

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Poor compression-4 stroke

Postby expo » Tue Aug 22, 2006 9:32 pm

I've got a Thunder Tiger F.91S 4 stroke that's almost a year old. When flipping it, the engine seems to have less than one third the compression of my buddy's .91 Surpass. It's only got 2-3 gallons of fuel through it. I've adjusted the valves to spec. with no increase in compression. The engine starts well and runs well but is not as powerful as other .91s. Something's wrong here.

What could cause the compression to drop with such little usage and what parts could I replace to increase it? Is there an easy test to check it? Expo
expo
 
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Postby Frank Klenk » Thu Aug 24, 2006 7:53 pm

Expo
If the valve adjust is good and they seat well then the only thing left is a stuck ring. Cleaning the ring groove and installing a new ring, provided liner is ok. I say new ring as removing and replacing the old one is tough to get on and off in 1 piece.

Frank 8)
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Postby expo » Thu Aug 24, 2006 8:04 pm

Frank,
Thanks for the reply. Do you think oiling it up with WD-40 or similar and letting it sit for a while would help matters?

If not, how would I get the piston out to replace the ring?
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Postby Frank Klenk » Fri Aug 25, 2006 7:12 am

Expo

You could try wd40, fill the cylinder. Let it sit for a few days, then run it and the carbon may blow out then. Try that and let me know what happens. Oh, and make sure the head gasket does'nt leak, that's another spot compression can sneak out.

Frank 8)
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Postby expo » Fri Aug 25, 2006 6:38 pm

I'll try it and let you know next week
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Postby expo » Sun Sep 10, 2006 6:53 pm

Frank,
I let it soak in WD40 for a week or so and found this out: The compression only seemed insufficient while loaded with after run oil. It seemed ok while WD40 was in it and while primed with nitro fuel.

That doesn't make sense to me since the heavier oil, not the thinner one, would have the viscosity to seal any gaps that compression gasses would be leaking out. Any comments on that finding? Tom
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