Rpm Drop Hesitation

[76chevy]

Recently got my quadrajet back on my 454 .030 over 10:1 motor tonight. After running from the key to the throttle a hundred times to get it to stay running, I finally got it to idle correctly with the help of a Rebuild Kit and idle circuit modification. It runs like a top now, but i have a new problem. When I rev it up, it take a second or two before it drops back to idle. I am wondering if this is a stuck power piston or maybe a vacuum leak? Any help is greatly appreciated!

[Frank400]

I'm guessing you've already checked/disconnected the linkage cable to take it out of the equation ?  Is it completely warmed up and "not on the choke" anymore ?

[Cliff Ruggles]

Hopefully you put one of our kits in it, the big block requires the high flow N/S assembly.

A vacuum leak can cause the idle speed to fluctuate some.

You also have the standard symptoms of light or weak distributor springs, or maybe one of those "goofy" aftermarket advance kits was installed? 

I've also seen engines do this running manifold vacuum to the vacuum advance and the vacuum is overcoming the spring in the advance at first then it settles down and timing retards and engine rpms decrease.....Cliff