What effect does a smaller spark plug gap have on emissions?

A smaller spark plug gap can lead to higher hydrocarbon (HC) emissions due to incomplete combustion. When the gap is narrower, the spark produced can be less effective in igniting the air-fuel mixture in the combustion chamber. This reduced spark strength can mean that not all of the fuel-air mixture burns completely, resulting in unburned hydrocarbons being emitted through the exhaust system.

While it is true that a smaller gap can also contribute to misfires, which subsequently increases hydrocarbon emissions, the option focusing specifically on the direct effect of emissions due to a smaller spark plug gap—specifically higher HC levels—accurately highlights the operational consequences on emissions that arise mainly from inadequate combustion.

Higher carbon monoxide (CO) emissions can also occur due to improper combustion, but the primary association with a smaller spark plug gap is more directly tied to higher HC emissions, making the option highlighting only HC emissions a clear choice regarding the direct effect of a smaller spark plug gap.