Two more things.

I'm going to assume something here, the car does not have an EGR valve, is this correct?

Second, one slim chance that it is ignition related. Pop the distributor cap off. You should have a four wire distributor module. Without removing the module slip off the yellow connector (IIRC) with the very small green and white wires on it. Take that connector and put the probes of a ohm meter to it. While holding the probes to the connector, move those wires in any and all directions and watch the needle.

If at any point the needle moves, even just slight wiggle, you have found your problem. That wire and connector leads to the pickup coil under the distributor shaft. Those wires are made to flex but under the "right" conditions they can break with no evidence on the outside that they are broken. If that is the case you will have to disassemble the distributor and replace it.

I'm gonna go with an accelerator pump as well. You asked how to troubleshoot it...you don't, just replace it.

Then, replace the Holley with a Rochester Q Jet. Yes, I am serious. Oh, but this book.

https://www.amazon.com/Rebuild...retors/dp/1932494189

On the accelerator pump Rocker you may find 2 sets of holes and corresponding holes on the casting. Many carbs that were built to fit a wide variety of engines will feature "tuning points" and the position of the pivot for the Accelerator Pump Rocker is one of them. What these holes are for is to adjust the Accelerator Pump Volume for the engine. If your pivot pin is in the hole set closest to the shaft of the accelerator pump it's in the Lower Volume position. Move that pin to the hole set further from the Pump shaft and you'll increase the pump volume. Many Many times just moving the pivot pin is all it takes to "cure" your stumble.

Did you buy the car or are you the one that stored it? More directly, did you ever drive it with that carb set up operating correctly? Depending on how the motor is built a 750 sounds like plenty of carb to me but I'm not an expert.

I had some of the symptoms you are having. Mine is a 327. I increased the octane level and it went away. Might be worth a try. My engine is much higher compression than I thought when I first got the car.

I am going to go against the grain and say a main jet problem. If it takes a second before it stumbles sounds like the accelerator pump is working and runs out of that gas before the main jets start flowing.
Are you sure the float level is correct? If too low then will take more airflow before they start flowing.
FYI I only ever worked on Quadrajets.
Maybe something with air bleeds for main jets or throttle valves not upen enough at idle and not enough air flowing through main venturies.

quote:

Originally posted by alreadydead:
Spark advance, timing is where I would look.

I second that.

A mechanical fuel pump, sitting for 10 years, is where I would start. The diaphragm is likely long shot. Replace the mechanical pump.

You can swap the springs in the vacuum secondary to tune it. You can also get different sized accelerator pump nozzles. Engine timing can also be an issue. Rear end gearing, stall converter, and overall weight even play a small roll.

You can have this issue with a double pumper as well.

quote:

Originally posted by c1steve:

quote:

Originally posted by alreadydead:
Spark advance, timing is where I would look.

I second that.

3rd

Further info:

Confirmed two streams of fuel when manually pumping throttle. I asked my brother (he is the one who did the carb rebuild) and he said it’s a new accelerator pump set to how it was when he took it apart. He’s the first to say he’s not an expert especially when talking about tuning.

He also relates he did not change the power valve.

quote:

Originally posted by Scooter123:
On the accelerator pump Rocker you may find 2 sets of holes and corresponding holes on the casting. Many carbs that were built to fit a wide variety of engines will feature "tuning points" and the position of the pivot for the Accelerator Pump Rocker is one of them. What these holes are for is to adjust the Accelerator Pump Volume for the engine. If your pivot pin is in the hole set closest to the shaft of the accelerator pump it's in the Lower Volume position. Move that pin to the hole set further from the Pump shaft and you'll increase the pump volume. Many Many times just moving the pivot pin is all it takes to "cure" your stumble.

I’ll look at this option. Thank you.

Answering more questions:

- I am the one that stored it. I drove it daily for years before storing. It developed this problem over time before I stopped driving it but it certainly wasn’t always there.

- I am running 91 octane as GM instructs for the ZZ4. It is a higher compression engine.

- relatively certain the floats are dialed in. On level ground a bit leaks out when you rock the car side to side.

So now if we rule out the accelerator pump I am seeing the following:
- fuel pump went bad over time (but remember it had this problem when I stored it.... so it would have to have developed before storing)
- timing is off?
- some sort of problem with the HEI that I had trouble following but will read again. Lol

I also noticed that one of the plug cable boots at the distributor is cracked. A new set of cables is probably in order but could that cause the issue?

I would say accelerator pump would be my first instinct.

quote:

Originally posted by thumperfbc:
I also noticed that one of the plug cable boots at the distributor is cracked. A new set of cables is probably in order but could that cause the issue?

If the wires were the issue you'd have sputtering or misfires throughout the RPM range. Since the carb was rebuilt I'm guessing it's either a misadjusted accelerator pump or a vacuume leak.

quote:

Originally posted by SgtGold:

quote:

Originally posted by thumperfbc:
I also noticed that one of the plug cable boots at the distributor is cracked. A new set of cables is probably in order but could that cause the issue?

If the wires were the issue you'd have sputtering or misfires throughout the RPM range. Since the carb was rebuilt I'm guessing it's either a misadjusted accelerator pump or a vacuume leak.

But remember, the car had this exact problem 10 years ago when it went into storage, before the carb rebuild.

If the problem was present before you stored the car.....

Many do not know this but GM had premature camshaft lobe wear problems during the early 1970's on their V-8's

Engine would give off all the symptoms of carburation / fuel pressure issues or engine timing advance issues.

I had problems with 3 different 350 V-8's - chased all the carb & timing potentials and also fuel pressure issues until I was blue in the face - In the end it was the cam lobes prematurely worn down - the bogging-down on acceleration.

A good friend on mine just dealt with this after spending a ton in diagnosis, new parts, rebuild of carb, and more diagnosis yet, etc.

I would make certain the camshaft lobes are at full spec.

I know this is a lot of tear-down, but will spare you pulling your hair out.

I am an ex-auto mechanic from the 1970's - graduated trade school for auto in 1975. Loved the muscle cars from 1960's into very early 1970's,

JoeSig

quote:

Originally posted by JoeSig:
Many do not know this but GM had premature camshaft lobe wear problems during the early 1970's on their V-8's
JoeSig

Brother in law had the same exact problem in a ford V-8 of the same era.

I wonder if there is any chance this was related to the reduction of Moly and other additives in oil to save the catalytic converters?

quote:

Originally posted by JoeSig:
If the problem was present before you stored the car.....

Many do not know this but GM had premature camshaft lobe wear problems during the early 1970's on their V-8's

Engine would give off all the symptoms of carburation / fuel pressure issues or engine timing advance issues.

I had problems with 3 different 350 V-8's - chased all the carb & timing potentials and also fuel pressure issues until I was blue in the face - In the end it was the cam lobes prematurely worn down - the bogging-down on acceleration.

A good friend on mine just dealt with this after spending a ton in diagnosis, new parts, rebuild of carb, and more diagnosis yet, etc.

I would make certain the camshaft lobes are at full spec.

I know this is a lot of tear-down, but will spare you pulling your hair out.

I am an ex-auto mechanic from the 1970's - graduated trade school for auto in 1975. Loved the muscle cars from 1960's into very early 1970's,

JoeSig

Cant be that as it’s not the original engine. It’s a ZZ4 crate engine from GMPP, manufactured in the 2001-2002 ballpark.

I have boxes of used Holley carbs in the basement and garage. I eventually discovered that the easiest and best way to fix a Holley carb was to replace it with a Edelbrock Performer carb.

So what is so good about an Edelbrock Performer carb and how long have they been out?

Does it have a vacuum advance in addition to the centrifugal advance? It is possible the weights on the centrifugal advance need to checked as working freely. They could be hanging up.

Also the vacuum advance may be bad and have a hole in the diaphragm.