New Q-Jet will not settle down to idle, it's driving me %$#%$#%$

[Rollo Grande]

1970 C/20 350 4v

My old carb backfired, spit flames, and sucked in every way...but it idled just fine.

Bought a brand new Quadrajet from a carb warehouse and it solved all of the big problems, but it will consistently not come down off high idle. I have to drive around blipping the throttle constantly to get it to calm down. It hangs up at 2500rpm, 1500, 1200, 1900, wherever.

I just tuned both screws using a vacuum gauge to 19 inches and set the idle speed around 800. Nothing changed.

Also this new carb is giving me 5mpg city, just like the old one did.

I suspect it might be missing a piece, that rich/lean clockspring thing, but they sold it to me as a complete plug and play "divorced choke" or whatever. I don't know that much about these things.

Any idea what to do?