There is now way a 150 motor is going to be started with 20 amps. It can't
be directly to the starter. I would think it goes to the solenoid. First
double check the wiring in the battery well. After that remove the small
wires from the solenoid and see if it still blows the fuse. Hint for trouble
shooting, use a large light bulb (car high beam headlight Ex.) instead of
the fuse. If it lights up bright you have a short.

Good luck.