smoke when shifting

i have recently rebuilt a 1990 350 tbi. it has about 285 hp, and after 3,000 miles i was racing someone and he told me everytime i shift that it would puff some smoke. so i was just wondering if this was normal for a slightly hi-po motor. its just a small puff, when you start the truck it does not smoke and you can rev the motor to as many rpm's as you wnat without smoke.

What color smoke? blueish white is oil, white is antifreze, black is gas. If it's black, it's just the extra gas in the throttle body slightly flooding the cylinders when the speed suddenly drops. Sounds like you won tho, if he saw the smoke :)

Also, did you confirm the smoke? Maybe he wanted you to have to tear into your truck chasing nothing.

Probably the valve stem seals. That's a sure sign of them being bad (if its oil smoke that is).

it is white and only when you shift at high rpm's. what makes me mad, is the fresh rebuild part.