Inliners International Forums General Discussion Engines 250 flat at full throttle

i have a 250 chevy with cam,hei, header,heated offenhauser intake, dual carter/webber carbs backed up with a t5 trans. the engine has great power at all speeds,plugs run a nice light brown color. the problem is if i floorboard the gas if falls flat on its face! if i backoff and squeeze back into it it pulls really well. id appreciate any help