Approximately a tablespoon of carbon buildup in a combustion chamber can raise the compression ratio half a point. A heavy buildup of carbon, like a little oil consumption can cause, can have a big effect on the tendency for an engine to pre-ignite the fuel, either from glowing carbon deposits that start small pockets of fuel burning before the spark plug fires, or in extreme cases, Dieseling due to compression ignition. Half a pint of very hot water, dribbled down the carburetor throat slowly with a fully warmed up engine running at a constant 2500 RPM or so can "steam clean" combustion chambers and reduce carbon deposits pretty dramatically.
Jerry
