I'm hoping that someone can give me a little knowledge on what may be going on with my 73 Nova.

BACKGROUND:

-1973 Nova hatchback with a ZZ4 crate engine, approximately 40k miles on the engine, which is around 16 years old, I think.

- the car went into long period of storage, from late 2007 until Sept 2017. It was only started 2-3 times in that period. The lat time it was started was about 3 years ago. It was allowed to run the tank dry.

- Prior to bringing it back into regular service the Holley 750 carb was completely disassembled, soaked in cleaner, and rebuilt using a freshly purchased Holley rebuild kit.

- Let's assume we adjusted the carb correctly, at least as it relates to idle speed, curb idle, and float levels. We used a vac guage as the manual calls for and I think we got it right

- fresh gas

- New rubber hose from the hardline on the frame to the mechanical fuel pump, and new lines from the pump to the carb. New fuel filter as well.

THE PROBLEM:

If you get on the gas even moderately aggressively, even when the engine is fully warmed, it falls on it's face for a second, hesitates, then catches up. It devoped this problem before it was put up for storage. Shortly before it stopped getting used the engine got new plugs and a new HEI distro cap. I hoped the carb rebuild would eliminate the problem, but it didn't so there must be another problem somewhere else, but I don't know enough to know where to start looking.

Any help?

Spark advance, timing is where I would look.

also check your accelerator pump.

accelerator pump working? OR pumping too much gas?

Did you replace your fuel filter?

Check your fuel pump and accelerator pump.

Ensure no vacuum leaks.

Did you replace the power valve in the carb? Mechanical or vacuum secondaries?

Check the advance mechinism in the distributor, mechanical and vacuum. Is the vacuum advance connected to the correct type of vacuum from the engine (manifold or ported)? Manifold is below the throttle plate, ported is above.

As long as compression and valve timing are good it should be something in fuel or spark.

If you haven't already replaced the power valve, I'd start with that.

Yeah, I know you said you rebuilt the carb, but that kind of stumble is more often than not an accelerator pump. Without running the engine, look down the carb throat as you stroke the throttle. There should be two streams of fuel pretty convincingly shooting out. And I know you said you adjusted the float level, but if there isn't enough fuel in the bowl, it will also stumble. The fuel pump, whatever type it is, should put out around 8 psi and pump a minimum of a pint every 30 seconds. (If the engine won't run that long with just the fuel in the bowl, that is a clue.) If all this is good, then look at the timing and advance. Pretty much what everybody else said.

Id lean towards accelerator pump. And verify the choke os fully open once at operating temp.

All of my suggestions have already been posted....accelerator pump, dist advance and timing, etc. I own a number of carburated autos, as I can work on them....not on FI cars.

Lean or rich tip-in may be visible on an aftermarket air-fuel gauge with wideband sensor. Innovate make one, as do AEM; there are probably others.

Data is a lovely thing...

Does it pop back through the carb?

Accelerator pump circuit.

I'm really not a fan of Holleys, I prefer a QuadraJet.

Yes, new fuel filter.

Vacuum secondary...

Seems the consensus is the accelerator pump is the first thing to check... how can I troubleshoot that?

When I stroke the throttle while watching I do see two streams of fuel shooting out.

And let me add this too as it might help... when I hit the gas, whether at idle or while cruising it doesn’t immediately hesitate, it starts to rev up for maybe half a second before falling for another second then it catches and goes.

Checking the accelerator pump is pretty simple. Remove air cleaner and manually operate the throttle linkage to wide open. You should see two streams of fuel shoot from the squirter into the venturi. It's mounted ahead of and in-between the front two barrels. The volume and duration of the accelerator pump can be tuned but the first thing is to check it's working.

Check the fuel bowl float level also, had one that had a pin prick leak in it and gave me major headaches before figuring it out.

Maybe the jetting is wrong due the change in gasoline formulation.
Have you tried some non-ethanol gas?
Also check that the distributor advance is working properly.

Accelerator pump. The diaphragms on the pump as well as pull offs and other dashpots loose vacuum due to the rubber failing from setting, with fuel, dry rot and such.

Look at the simple, obvious and "free" solutions first.

Is the stumble at WOT or PT?

Those Holley's with vacuum secondaries have a lag when transitioning to WOT. That is why I prefer a mechanical secondary carburetor.

I had a virtually new 650 Spreadbore vacuum secondary that the only way I could make it dump the back barrels correctly was to put a machine screw and nut into the secondaries linkage as a stop to positively open them up. I got tired of screwing around with the diaphragm springs and checking adjustments until I was blue in the face with no improvement. Secondary engagement would occur but it was a smooth opening, not the instant four barrel "dump" that the car needed.

Try disconnecting the vacuum hose at the distributor. Could be broken wire at the HEI distributor pick up coil.