I have a 1969 Firebird with a rebiult quadrajet on it with no numbers. When I bought this car it had a 1974 built up 350 ci in it. I cloned my bird into a trans am and switched out the 350 for a 1970 400ci 350hp out of a GTO. The motor and parts sat around for about 4 months waiting for the car to be repainted. I used the carb, distributor, coil and fuel pump from the 350 because the 400 did not have these parts on it. After putting the engine in and running it I found out that when the rpm's got up to 4000 the engine starting bucking. I talked to several people and I was told many things that it could be, from vacumn to valve springs to distributor to carberator. I had a mechanic let me use a quadrajet that he had sitting around. It had several vacumn lines on it that I capped off. I ran it and the engine did not buck at all. It seems like this carb does not have as much torque but the bucking was gone. What do you think the problem is with my original carb that bucks?

Thanks
Greg