Blows Main Fuse at 1/4 Throttle???

Hey guys, anybody had this problem before??

Took my son out for a ride with the new motor. Dang it hauls ***!!
Anyway, all was going good and we were headed home I started to get on it in second gear and at 1/4 throttle it blew the main fuse that is 30amp.

Found the problem instantly put in a new fuse cruzing slow and up my street started to roll into it again and it blew another one. So replaced it and pulled in the garage and for giggles with NO LOAD IN NEUTRAL at 1/4 throttle it blew another one.

It fires right off immediately when I replace the fuse. Never had this problem before!

I have checked and nothing is hanging up on the throttle cable, no bare wires nothing that I can see.

Your thoughts???