X2 motor starting issues

[itr0754]

Well, I just installed a brand-new blue Keihin 42 carb on my X2. Everything runs as sweet as can be expected except for one thing. Every once in a while, when I start the engine, it will fire right up but it will fire with a loud bang and then continue to run normally. I have 150psi on my cyls with a Westcoast head (30cc domes). What could the problem be ?? Will running low-octane gas cause this ??