I built my first 350 and i'm having problems with backfire through the throttle body. this is a motor out of a 93 k1500. i punched it out .40 over and replaced the stock heads with 96 vortec heads. it has a truck and rv cam also vortec carb. intake adapted for the stock throttle body. the motor is in time and valves are adjusted. could this be a fuel pump problem or something else? i would be grateful for any advise. thanks