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I have a 1943 GPW with a Wilson rebuilt starter, new 6 volt battery, new starter switch, but the starter will still just won't turn the engine over. The only way I can get it to turn the engine over is to use a 12 volt battery ; then it turns the engine over really well. A load test has been done on the 6 volt battery and is supposed to be good. Also, the starter has been taken apart and has been tested for shorts but seems to be in good working condition. Would anybody have any ideas why this starter won't turn the engine over ; could it be a 12 volt armature or wrong fields ??? I've seen an article that mentions rebuilt starters giving this kind of problem. Should I just buy an original starter ???
Thank you, Tom

I would start with a professional automotive electrician bench checking the current starter. A lot cheaper than just buying new parts until the problem goes away.

Carefully feel around for overly hot spots after you have tried to crank the engine....especially around the floor starter switch & all grounds.
Bypass the grounds by directly connecting a heavy gauge jumper cable from battery ground to the starter.