Maybe someone here could add more info here as to the spark plug range that you could try. I have heard that changing the induction system and changing the exhaust system also changes the requirments of the spark plug heat range.
The way I understand it is adding more carbs and better exhaust flow will cool the engine somewhat and require a cooler than stock spark plug to be used. Maybe you could be running spark plugs that are to hot for your engine causing your engine to run hot.

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