Sounds like to me you have either a completely shorted primer solenoid OR a shorted wire going to your priming solenoid. Because when you push in the key switch, it applies voltage to the priming solenoid and if that blows the fuse, it is applying a straight voltage to ground or the engine case, and that causes the circuit to use too much current and blows the fuse. I'd check that first. Because if you continue doing that, you will also need to replace the ignition switch as well... JMHO!