A number of things can cause the main fuse to blow every few minutes. I would look for any chaffed wires rubbing on the frame or motor somewhere first. While you are at it, pull apart every plug connector and look for corrosion and clean the two grounds and both of the battery cables. Then check the ignition switch... feel the switch body to see if it is getting warm...? Then check the kill switch (has it gotten water in it?) and use your multimeter to check the regulator/rectifier. You should gain a clue or two by that time....