Another side effect of carbon accumulation is increased compression pressure. If an engine is running close to the pressure that would require a higher grade of fuel, say 9.5:1 or so for a regular-gas engine, a couple of tablespoons of carbon accumulation in the combustion chamber will boost the compression enough to get some detonation. A few cylinders trying to fire too soon and drive the piston backwards will definitely affect performance.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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