Bert is correct
to elaborate
if you turn the key to the run position, certain circuits are now potentially live with battery voltage. this could head lights, electric clutch battery ignition. when you turn the key to the start position, the previous circuits are active as well as the start circuit.
if the fuse blows when the key is in run, you to check all wires coming out of the switch to the various loads. if it only blows when in the start position, then you can eliminate all of the other circuits and concentrate only on the start. you should have a schematic/drawing and follow the wires. do not jump around as you will miss something.