It blows large clouds of white smoke when I start it up, then it won't smoke toi bad unless I go over 40mph or the car is
real hot. When the large smoke clouds around 40 mph the car makes a loud knocking noise in the engine??? I've been driving it for 3 weeks now but only backrods. I had a complete tune-up done and a new distributer cap.

 

sounds like u might need to rebuild the motor
could be the rings or something like that

 

I'd say rings or a head gasket. Check your coolant and see what it looks like.