What can cause deposits on a spark plug?

Deposits on a spark plug can arise from several factors, both of which contribute to combustion inefficiency and lead to the buildup of unwanted materials on the spark plug electrodes.

Improper carburetor adjustments can lead to an incorrect air-fuel mixture entering the engine. When the mixture is too rich, more fuel than necessary enters the combustion chamber, leading to incomplete combustion. This results in the production of carbon deposits, which can accumulate on the spark plug, impacting its performance and longevity.

Similarly, an incorrect fuel-to-oil mixture can also contribute to deposits. In two-stroke engines, for example, if the oil content in the fuel is too high, it can produce excessive smoke and carbon during combustion, leading to the formation of deposits on the spark plugs. On the other hand, a fuel that is not mixed properly can cause similar issues by changing the combustion characteristics.

By identifying that both improper carburetor adjustments and incorrect fuel-to-oil ratios can cause deposits, it's clear why the answer is both of the above. Each factor plays a crucial role in maintaining optimal engine performance and ensuring clean spark plugs.