At a loss - engine stalls under load

[R68GTO]

Looking for some insight on troubleshooting my engine troubles.  Car starts, idles, drives out fine until you give it any heavy throttle.  I'm not talking mash to the floor, just hard acceleration, say 1/2 to 3/4 throttle.  When you do that, engine stumbles, then dies.  Quickly restarts and runs fine at light/partial throttle.  Car will rev in neutral without issue.  Car also seems to be able to handle heavier throttle when I first take it out.  After about 1 mile of running through the gears a few times, it can't handle heavy throttle.  I started to troubleshoot, then fell into the black hole of replacing parts one at a time to no avail.  Here's what I've done/checked:
1) checked plugs - fine
2) check resistance on plug wires - fine
3) checked coil - fine
4) replaced points with electronic module / along with coil
5) checked timing - fine
6) plugged vacuum advance - no change other than difficult idling
7) changed plug wires
changed fuel pump

What am I missing?  BTW, it's a solid lifter 427 built to stock specs - heads, cam, carb, distributor are all OE stuff.

[Steve68]

Have you checked the float levels and other carb adjustments such as the accelerator pump?

[jwbavalon]

I'm with Steve on the accelerator pump.  On the "first take off" the choke is on sop lots of extra fuel.

[R68GTO]

Float level is good in both bowls. 
How do you tell if accel pump is functioning correctly?

[GMAD_Van Nuys]

The fuel inlet in your gas tank could be clogged with debris.

[Steve68]

Float level is good in both bowls. 
How do you tell if accel pump is functioning correctly?

Engine off. Remove carb cleaner. Look down carb throat. Use a light to light up area in throat if you can’t see clearly. Hold the throttle linkage in you hand and as you look down carb move the throttle as if accelerating, there should be a discharge (squirt) of gas into the throttle body area.  If you don’t see gas coming out during the throttle movement then there is an issue and likely your problem.

Use a service manual or Google to see how to set the correct spacing between linkage and pump lever.

[R68GTO]

Checked the access pump per Steve's suggestion....carb received a shot of fuel as soon as I moved the throttle lever.
Fuel tank is new as are the lines all the way to the carb.  I did retain the original sending unit just because it was original to the car.  It was in perfect shape and the filter sock was still pliable and not deteriorating.

[Steve68]

How did you check the fuel bowl levels?

[maroman]

What kind of carb?

[R68GTO]

4346 GM Holley - all original parts, Erik Jackson went through it before I installed it.

[David K]

Did the problem occur out of the blue?

[Stingr69]

Fuel starvation sounds likely.  Filter inside carb fuel inlet needs to be looked at first.  Also, currently available fuel pumps seem to be pretty bad.

You might want to mount a fuel pressure guage at least temporarily to see if you loose pressure. 

[169INDY]

 Car will rev in neutral without issue. 

I had a problem like this on a quadrajet car, turnout to be missing that darn thin metal separator gasket.

Check the vacuum values?

Might NOT apply to your Holley situ

[R68GTO]

Did the problem occur out of the blue?

I have been trying to recall the sequence of events.  I believe this condition started after I installed new 8.5 power valves in the carb.  I did check them before installing.

[R68GTO]

Car will rev in neutral without issue. 

I had a problem like this on a quadrajet car, turnout to be missing that darn thin metal separator gasket.

Check the vacuum values?

Might NOT apply to your Holley situ

I'm not familiar with the thin metal separator gasket.  Where is it on a quadrate? Vacuum is right at 14.5 inches