I have been running a 6V starter on 12V for 25 years or better. I think you must have another problem. I recently switched to solenoid start and put a 12V solenoid on my 6V starter and I believe that to be sound advice. A 6V solenoid I am told will hit flywheel too hard on 12V and damage ring gear teeth. Usually, with a 6V starter on 12V the engine turns over fast enough that it is running before starter has to turn much at all. Could you have bad battery/starter cables causing too much current?
