Looking to get some help, I’m new to this so bare with me lol.
I just picked ip an 85 s10 with a Chevy Sbc. It has been modified and I’m looking to get it fired up. When I purchased it the guy told me the starter needed to be shimmed to get going. There is an ignition salter switch on the console. So I shimmed the starter and got set. Truck turns over and cranks but does fire. So I flick on the switch and and turn the key for a few seconds an it cranks but they the starter kicks loose and makes a grinding noise. Almost like it throws too much power ?? So I go under the truck tighten it down and crank with the switch off and it tries to turn over and no grinding. But when I flip that switch it does it again. Any tips or advice would be greatly appreciated .

 

1: Go get a NEW starter
2: Verify wiring (sounds like you have a short/double power feed to your starter and it is engaging once the engine is running)
3: Install new starter.

It seems to me that the starter is getting knocked loose because it is engaging the flywheel while the engine is running. Check the teeth on the pinion of the starter and the flywheel. I’d be a paycheck that it ain’t pretty.