One tablespoon of carbon buildup on top of a piston will raise the compression about half a ratio, but it won't be consistent from one cylinder to another. The carbon deposits can also glow red hot and promote pre-ignition and detonation on a hard pull. Take an air die grinder or even a high-speed electric drill motor and a fine-bristle wire brush and buff off as much carbon as you can. Just be sure to blow the residue away with lots of compressed air after you finish cleaning.
Jerry

Last edited by Hotrod Lincoln; 02/13/2010 4:29 AM.
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!