What usually leads to an increase in firing voltage?

An increase in firing voltage is typically associated with higher secondary resistance in the ignition system. When the secondary resistance increases, the ignition coil must generate a higher voltage to overcome this resistance and deliver a sufficient spark to ignite the air-fuel mixture in the combustion chamber. This higher voltage is necessary to ensure that the spark can jump the gap at the spark plug, especially in conditions where the mixture is lean or the engine is under high load.

In contrast, conditions such as a crack, fouling, or contamination of the spark plug can actually hinder spark transmission and may result in a lower effective firing voltage. A crack in the spark plug can cause a loss of voltage, leading to misfires. A fouled spark plug can short out or create a weak spark, resulting in inadequate voltage for ignition. Similarly, a contaminated spark plug could result in poor conduction, which means that instead of needing a higher voltage, the ignition system might struggle to produce any spark at all.

Overall, an increase in secondary resistance necessitates an increase in firing voltage to maintain effective ignition, which underscores why this is the correct choice for the question.