High Compression Problem Follow-up

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Bobwombat - 07/13/09 at 3:33pm

I posted recently about unusually high compression (approx 190) on my Ace 100....
Well, I popped the lid & found the most carbon buildup I'd ever seen on a piston (777 original miles). But, piston-to-cylinder clearance was excellent at .004 to .005. So, I polished the piston dome & cylinder head combustion chamber, installed new ringsm etc & re-assembled.
Result? 162 psi compression. Somebody please tell me this is a good result! Can I be reasonably convinced now the excessive carbon build-up caused the dramatic increase in compression? My son-in-law (Master H-D Technian) said he sees this often.
Thx, Bob in AZ

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thrownchain - 07/13/09 at 7:53pm

Carbon build up will cause a compression increase, the build up makes the chamber smaller, and you're trying to pack the same volume of air/gas mix in there, so the pressures will be higher...

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hodakaronwa - 07/18/09 at 11:55am

Carbon buildup will most always will increase compression and not always good.
EXAMPLE: I purchased a 1972 Chevy truck with a 250 inline six cylinder new in that year. I drove that truck for over twenty years all over. "The best Chevy I ever had" as far as an engine goes.
Anyway! over the years the compression increase gradually, in order to continue to burn regular gas i had to gradually pull the advance out of the initial timing. "Retard IT" over the years and eventually the initial was set to "ZERO" and it still pinged and rattled.
It had 179,000,00 miles on it by then. I gave the engine to a fellow that wanted it for an old 56 as it still ran great and was tight consuming minimal oil. The fellow pulled the head and removed all the carbon from the piston crowns and the combustion chambers then assembled the engine with new gaskets. It ran just fine with the OEM timing after that.

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