I have a model B tractor and my starter will not crank the engine. I took the starter off the tractor and with jumper cables, (it has posative ground) it spins over nicely on my bench. When installed I hooked the jumpers directly to the starter and it will not crank the engine over, just a "grunt" sound from the starter. The battery is fully charged, has new cables and everything looks to be clean inside the starter. The tractor was sitting in a garage for 5 years prior to me getting it and the tractor starts well when I use the crank start. What could be wrong? Thanks Donedeere in New York.