I recently purchased an 84 Chevy pickup with a supposedly recently rebuilt 350. The 350 is approximately a 92, which has had a performer intake and quadrajet and 84ish HEI distributor swapped onto it. It runs pretty well but has a pretty bad vibration at idle. As soon as it gets slightly above idle the vibration diminishes and by about 900 to 1000 rpm it is completely smooth. I wasn't too worried about the vibration because I bought the truck so cheap that even if I have to build the motor to get rid of the vibration I'll still be in the truck cheap enough.

I'm wondering if the vibration could be a tuning issue, since it goes away right above idle, or if it has to do with the recent rebuild ie pistons way different in weight than the originals etc.

Any ideas/thoughts?