An RC car engine's idle gap is the width of the carb opening when the throttle is at idle. It controls the amount of oxygen that is allowed into the engine to mix with fuel when the engine is idling.
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Settle in for this one - it's over an hour, and has great production value (and is hilarious!) But if you want to know how a carb works, how the needle valves work, how to adjust your idle, or anything else involved with nitro tuning, you must check this out.