I have a '73 with a 350/TH400 and quadrajet 4bbl. When I floor it and the trans kicks down, there is a hesitation (RPM's drop and it sputters a little) then it takes off like it should. I get a backfire a few seconds after I lay off the pedal, too. It'll do this once or twice after I start it up. It doesn't seem to matter how long the car has been running, it'll do it hot or cold.
Hate to say it, but this is the 1st 4bbl I've owned and I'm a little baffled by it's behavior.
The Vette used to belong to my dad and after he passed away, my mom held on to the car for a few years. She pretty much just drove it around the block every few weeks and would let it idle for 15-20 minutes. Not sure if that would have anything to do with it.

Any suggestions would be greatly appreciated!

 

I think the seal on the accelerator pump is worn, damaged, or has fallen off the pump rod (internally). It doesn't sound like you are getting a good squirt out of the accel pump.

You can check this with the engine OFF by removing the air cleaner, holding the upper choke plate open and looking into the primary venturi. Use your hand to exercise the throttle fully and you should see a good stream of [liquid] fuel going into each primary bore. If the shot is weak or you see no stream at all, the accel pump seal is your problem.

 

Checked it out and it looks to be getting a good squirt of fuel in the primary. It pushed fuel for about 1-2 seconds.
Any other ideas?

 

The secondary air valve (spring loaded upper flap over the secondary venturi) can be adjusted [spring tension] either too weak or too strong and get the same result. The best starting point for that airvalve is to loosen the lock/setscrew {underneath the right side shaft pivot}, back off the spring so it is totally loose then turn CW until it just removes the slack and starts to apply tension to the spring, then turn another 3/4 turn CW and lock down the setscrew. At that position, place a mark on the outside of the shaft pivot to indicate the alignment of that screw slot for future reference.

Now, try it out at WOT. You only need to go through one gear; you can do 2nd gear if you don't want to wear your tires away (). If it bogs, choose a direction you want to adjust the tension screw and turn it 1/8 turn. Try it again. If BETTER, go another 1/8 turn in the same direction. Continue until it gets a bit worse, then make final adjustment 1/8 turn in the opposite direction (to reverse the last move).

If the first test is WORSE than the initial test, turn the screw 1/4 turn in the opposite direction, then test again. If better, continue in THAT direction until it gets a bit worse, the reverse the last adjustment and LOCK IT DOWN.

There can be other causes of your problem, but you should rule each one out...or maximize the performance of each "adjustment", before you try another possible cause. This will prevent you from having to go back and re-adjust all the incremental improvements you make along the way.

Try this out and report results....good or bad.