i bought a motorcyclecarbs jet kit. i have been getting horrible mileage. so i deciede to jet it a little leaner as the plugs were black. i had a 150 mikuni main with one thick factory washer and 2 small shims on long end of needle. now i have taken one shim off. so one thick one and one small one on there now. and have stepped down to a 140 mikuni main jet. bike starts idles fine, run good till about 1/4 to 1/2 throttle, then it starts cutting out and will barely pull on full throttle. any ideas????