'79 350 idle problem

Need some advise on idle problems with my '79 Blazer.

First of all, this has the original 350 with many upgrades (Edelbrock Performer manifold, RV cam, lifters, springs, roller timing chain & gears. FlowMaster 3" exhaust).

It has the original Quadrajet (rebuilt & tuned to the Edelbrock after market parts above).

It has good vacumn (EGR & PCV work fine), fresh NGK plugs installed, and timed. Does not burn oil (replaced the valve seals recently) or have contaminants in the oil (i.e. head gasket issues) and generally runs strong, except for this recent idle problem.

It seems like a vacumn problem, but I can't find any leaks! Again, this motor purrs like a kitten, when rev'ed up from idle.

But at idle, it lobes like it has a radical cam, and the idle RPM's vary by ~ 50-75 RPM's (which it just recently started doing).

Any idea's what to check next?

You might want to check and see if you have all of your plug wires connected ;)

i'd check the vacuum advance on the distributor, sounds like maybe its stuck in the advanced position. also if you have an EGR valve, check to be sure its not stuck open, even the slightest bit open will effect idle.