here's what I have: 68 400, comp cams 280ah-10 (.481 intake/exhaust),comp cams 1.65 ratio roller tip rockers, edelbrock performer intake, 650 holley 4150 dbl pumper with 71 jets in front/76 jets for the secondary's, 4-speed, non-a/c, msd billet distributor mech advance, 6-al ignition box, msd starter saver. right now I have the initial advance set at 18 degrees with the heavy silver & blue spring in with the black bushing. the firebird starts fine, idles fine...idles around 850-900 rpm with no issues. now, when I go to drive it & just give it minimal throttle it does fine. when I punch it, the engine revs with no problem, tires break loose, then it just dies. period. now, there's still electrical power but no "go". if I leave up on the throttle, then ease back into it, then I have power again. I just bought the holley last year, had it worked on by a tuning shop here because I thought it was the carb. the carb worked fine on their test engine in the shop after they finished it, no throttle loss at all when they gunned it. my guess is ignition timing? I just don't see this carb being to big for this engine set up. back off the initial? is it getting to much advance to quickly? anyone ever have this issue?

I should add with the 1.65 rockers, lift is now .531 for intake & exhaust. I just had the motor completely rebuilt, valve to piston clearance is fine, new springs as well.

sounds like its running out of fuel. Do you have a fuel pressure guage you can T into the fuel line @ the carb? also the stintered bronze fuel filters that Holley has used on the inlets of the float bowls can gum up quite easily. Try removing them and see if it changes anything. I don't know enough about msd stuff to diagnose your electrical but others here do. Good luck.

does sound like a fuel starvation issue, BUT, on that 6AL box, do you have the limiter set too low?

I've been thinking about fuel starvation too. i may put a fuel pressure gauge in just to check. i have a 5500 rpm chip in right now, i'll stick a 6000 chip in it tomorrow just to make sure. i'd be shocked if it's the rev limiter. I've heard the msd rev limiter cut out an engine before, where you could still hear the engine, but this is just completely cutting off the engine with my foot still in the gas. that's why i'm totally stumped on this one.

Sounds like fuel to me s well. If the carb has an inlet fuel filter I'd toss it and put an inline one between the pump and the carb.
It may be the fuel pump as well. Mine was quitting a couple of years ago and someone from this site suggested I put a gauge on and try. I taped a gauge to the windshield wiper and yes the pressure dropped just as the engine quit. I put on a new pump and that solved the problem.
If there are some rubber hoses connecting the steel fuel lines between the tank and the pump getting soft they could be collapsing and stopping flow.
The screen over the pick-up in the tank may be clogged as well.

Did you check your float level? Maybe too low to feed it once you open it up.

a kink in fuel line?

I ordered a holley 110 gph mechanical fuel pump this evening. I have the 80 gph pump on it right now. I've had numerous people tell me that with the mods to the engine the current fuel pump isn't getting enough gas to the carb. I did check the float level tonight & it was spot on. the pump will be here Thursday, i'll get it mounted Thursday after work & see if it fixes the problem.

All the other engine build parameters are irrelevant. Even if it were the 6AL box, and when the RPM's hit 5100, 5200, 5300 etc, that break up would definitely be noticeable and consistent.

It is a fuel starvation issue. I am 23% sure of it.

x2 After my 350 HO was rebuilt I would run out of gas half way thru 2nd gear. Put on electric pump and problem fixed.

Hate the noisy pump though. Just get a proper sized stock pump.

Sometimes the fuel pumps act up for a few weeks before they quit.
Try a new one.

I was told more than once to get an electric pump as the mechanical wouldn't keep up, but the mechanical pump I put in a couple of years ago feeds my 428[463]cid engine at 6ooo rpm.

Another consideration while you're messing with the pump swap: How large are your fuel lines? ALL of your fuel lines. All the pumping power in the world still won't suck a golf ball thru a garden hose any faster...

Depending on what your car was born as, most of these cars had 5/16" fuel lines in various locations between the pick up in the tank and the carb. Not to mention 45+ years of mechanical hacking history. A single inch of 5/16" line is a MASSIVE restriction in fuel flow.

3/8" should be considered a bare minimum from end-to-end, in order to maintain enough flow to feed a 400 with any mods whatsoever.

Sorry it took so long to respond. Between my job, my wife's job, & an 8 month old baby girl, free time to work on the cars is almost non-existent these days. Thank you for all the input though. I did finally get it fixed. I had to install an electric fuel pump & that did the trick. The engine doesn't die at all. I put a fuel pressure gauge in just to see what was going on & the pressure dropped to one psi when I floored the pedal. That's when I went out & got the electric pump & now there's pressure. Somewhat of an easy fix, but it's done.