What effect does advance timing have on HC levels?

Advance timing in an engine refers to the adjustment of the spark plug's ignition timing to occur earlier in the cycle of the engine. When timing is advanced, the fuel-air mixture burns more efficiently, leading to more complete combustion. As a result, the production of unburned hydrocarbons (HC) is reduced, which directly affects HC emissions.

With advanced timing, fuel has more time to combust before the piston reaches the top of its stroke, allowing for greater thermal efficiency and a more complete burn of the fuel-air mixture. This efficiency reduces the amount of HC that escapes unburned into the exhaust, leading to lower HC emissions.

In this context, while some engines might experience different behaviors based on specific tuning or conditions, generally, advancing the ignition timing would lead to no change or a slight decrease in HC levels due to improved combustion efficiency.