vibration at idle

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?

it could be the fly wheel .if the pistons are a match set it would not do that even if abit heavy

I had a badly balanced torque converter do that once, but I can't remember if it smoothed out just above idle or not. Wrong flywheel (flexplate) is probably a likely possibility.

Had an '84 pontiac that I chased a low speed miss on and finally took it to a guy who rebuilt the carb. Still had the miss when he fired it up again. Turned out to be the fuel - the car did not idle on cheap ethanol blends. Tank full of Exxon was all it needed.

I would be sure of your basics - plugs, base timing, fuel delivery, etc before chasing mechanicals. You may have already exhausted all of those, but you did not say...

the 1 pc seal cranks SBC are external bal i would look and see if it has the weight on the fly wheel