What all is running on the ignition fuse. Add up all the parts (Ignition,maybe fuel pump etc etc) and see what they come to. At startup some items like the fuel pump will surge past their constant rated amp draw. If it just blows right away and has since you put on the kit you have a dead short. If it only blows sometimes I'd say you are over taxing the circuit. The 20 A fuse is there to protect the 20A wiring a 30 A fuse may or may not allow the wiring to heat up so that wouldn't be my first choice, either way it shouldn't do that.
M5