Hey Chevy Power, I got a QJET on my Monte. I've been thru 3 of them over the years. For some reason when they get old, they start to hesitate, bog the motor, all that fun stuff. I've just rebuild'm and that fixes a lot of the problem. I also ran an Electric fuel pump that help out by suppling constant fuel pressure. On some the Vac secondary if equppied can be opening to quickly, or too slowly. Usually they had mecanical secondairies. right now I had a vac secondary, and it works great, the best so far.
If It's a lighter car or lower gearing u want them to open faster, but if it's over 4000lbs or so, having it open a little slower will keep the powerband steady over the curve...

also, dirt is its worst enemy.... just a little between the needle and seat can hose you... So A good way to clear it is to rev it up , then cup your hand over the primary butterfly till the engine almost stalls, this causes a great deal of Vacuum to be pulled thru the carb, in most cases clearing out dirt and other crap in there.