It could be a leak. Check your carb. What engine are you running? Also, what are your needle settings? Your high speed needle should be about 3 turns out to get it started up, and the engine should be tuned richer when you apply throttle. Your mid-range needle should be flush. It depends on what engine you have though. I had the same problem with one of my engines a long time ago, but I just had a leaking carb. I fixed it in a second and ran the car the whole day without anymore trouble.