Do you have a good spark from the mag in both full advance and full retard??
Remove both spark plugs and put a “test” spark plug on one of the leads (but don’t leave the other lead in thin air – either earth it or attach another plug) and open the “test” plug a bit at a time observing the spark being generated in both advance and retard positions. If you get a good spark on full advance with a relativey large gap but not on full retard, suspect the mag.
This is because a “poor” mag can give a lesser spark as the cam ring is rotated to retard the ignition timing – as providing an advance/retard by this method is a bit of a compromise anyway. Allow me to explain.
Disregarding the actual ignition timing (i.e. when the spark occurs relative to piston position), the mag has an “internal” timing – which is where the armature is relative to the magnet(s) to provide the greatest voltage being induced into the primary winding of the armature as the points open.
The magneto is designed to give the optimum internal timing at full advance, so by moving the cam ring to retard the ignition also alters this “internal” timing – so if the mag is a bit dickey, it will give a poorer spark with the cam ring in the retard position. That might be the cause of your spitting back. Just a thought.
Well thats my story and I’m sticking to it.
Stan Thomas.
Korean Washing Machine “V” belt and Pulley Museum.
Penkridge.
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