Pop the air box cover off and take a look for 2 hoses coming in just outside the air filter. If they're there, see my post about relocating the breather hoses. Basically those hoses are putting un burnt gasses back through the engine, its a cheap emissions control. So those gasses are mixing with clean air and causing the backfiring and missing. At least that was the case on my XB12. I'm not sure if the blast has the same set-up, but if it does, thats the culprit.