What effect does an excess of carbon accumulation in the combustion chamber have?

An excess of carbon accumulation in the combustion chamber typically leads to an increase in the compression ratio of an engine. The compression ratio is the ratio of the cylinder's maximum volume to its minimum volume during the combustion cycle. When carbon builds up on the combustion chamber surfaces, it can effectively reduce the volume of the combustion chamber. This decreased effective volume increases the overall compression ratio. A higher compression ratio can often lead to improved performance and efficiency, as it allows for better ignition and combustion of the fuel-air mixture.

However, it's essential to note that while increased compression can lead to some performance benefits, it is not always advantageous due to the potential for knocking (pre-ignition) and other issues that might arise from too high of a compression ratio. Nonetheless, in the context of the question, the accumulation of carbon contributes to the increased compression ratio due to the physical space it occupies in the combustion chamber.