Dieseling (Run-On)

Dieseling means the engine keeps running unevenly for a few seconds after you turn the key off. With no spark, the mixture is lit by hot spots in the combustion chamber – glowing carbon deposits, an overheated spark plug or simply a very hot engine – like in a diesel.

It mostly affects carburetted engines, especially when idle speed is set too high or the mixture is lean. Fuel-injected bikes cut fuel along with the spark, so run-on is rare.

Fixes: set the idle speed correctly, check the mixture, use the right heat-range spark plug and remove carbon if needed. Persistent run-on is a sign the engine runs too hot – check cooling and timing.