Lowering the idle speed is sometimes effective in masking the problem, but idle speed is not the cause. Something hot in the combustion chamber is causing the mixture to ignite in the absence of spark. Look for a lean mixture (vacuum leak or carb settings), marginal cooling system, carbon deposits, or too hot a plug. We run old 6 cylinder gas gensets that shut off very politely from 1800 RPM by simply killing the ignition.
