Your starter is just a motor like your circular saw only DC current. If you have used jumper cables and bypassed the solenoid running power to the starter, check if there is a ground problem. Also run a wire directly from the body of the starter to the negative terminal of the battery. If it still doesn't turn over you need to send the starter off to be repaired.
The starter on my 210b did that. I took the starter off and wired it to a battery on the bench and it worked and put it back on the tractor and it didn't. I solved the problem by cleaning all the rust, grease and dirt that was between the starter and the block.