What do you mean by big bore? is it a 404? Or is it just overbored .040" or something similar? Anyways, your jetting sounds a little lean, depending on elevation, and temp. Now, I had a problem exactly like you describe. It turns out that my primary coil wire, (the orange one) had come loose on the terminal. my bike started and ran fine, you got on it, and once it should be in the powerband, it would spit, sputter, backfire, and do all sorts of crazy shit. Also could be a cracked reed......