Timing is not an issue when the key is off. There is no timing involved. THERE IS NO SPARK when the KEY IS OFF!!! REPEAT THE IGNITION SYSTEM IS DEAD WHEN THE KEY IS OFF. Its caused by to much air/fuel hitting hotspots in the chambers and combusting. The air/fuel excess is caused usually by the throttle blades being opened to far in an attempt to bring the idle speed up so the engine wont stall. This usually needs to be counteracted by (contrary to what most everyone here is saying) increasing, YES, INCREASING initial timing so that idle speed is brought up and throttle blades in the carb CLOSED down to stop a surge of fuel being pulled in at idle. Or by using an idle solenoid which releases/retracts when the power is off, there by allowing the throttle blades to close cutting off the air flow, there-by stoping any vacuum signal to the carb and stoping the fuel flow.