Looking for some help with a Holley 770 truck avenger on my mild 440 , 4spd in a Ramcharger. Starts, idles great. Runs really strong off idle, feels like I could burn my tires off my wheels if I wanted too, but as the rpms come up ( I don't have my tach hooked up but about 2500-3500 rpms) engine goes flat with a little stumble and no matter how much more I press the gas it will not rev anymore. As Holley tech support has recommended I've tried different secondary springs and different PV's 2.5, 6.5, 8.5 all with little to no difference. I have other carbs (Holley and edelbrock) all work fine, so I don't think its a ignition issue. Anyone have any idea what else I can try, before I pull this thing and turn it into an expensive paperweight.?

New problem since switching to this carb? Sounds like possible fuel supply issue. percolating, vapor lock etc.

no vac leak from a base gasket. float levels correct. no restricted inline filter hidden in the inlet fitting. wouldn't think a SA would be lean but might bump up the primary jets 4 sizes for a quick test.

Do you have a possible linkage issue? Since the other carbs work I would focus on what's different about this carb.

Silly question as I got stung by this many years ago, try it without the air cleaner, In one case it was too small, in the other, the bottom of the lid was too close to the inlet of the carb.
Another thing to try is wire the secondaries closed. it could help isolate the problem

""but as the rpms come up ( I don't have my tach hooked up but about 2500-3500 rpms) engine goes flat""

At this rpm range, you are cruising on the MAIN CIRCUIT. Test larger main jets and see if that makes a difference. If your mains are too lean, you will have a stumble in this range when changing rpms.

how much pressure is fuel pump putting out?

agree with this.
jet it up atleast 4 jets sizes on the primary side (doubt it has jets in the secondary side).
the correct power valve will also help tremendously with part throttle high load driving. start there with a PV rating half of what yr idle vacuum is. my truck idles at 18", i went from a 6.5 to 8.5 PV and it made it much more pleasurable to drive.

basically, get the PV right, then get yr idle off idle mix right. then work on the primary jets. then work on yr secondary spring rate...

Thanks for the reply's. Yes, its a new issue. Other carbs run fine but need rebuilds(leaky). Float bowls set correctly. Steady 7 psi. Linkage opens to WOT. I have tried no air cleaner at all, no change. 15" at idle and it does have secondary jets. Off idle is great. There does seem to be a lot of black fuel fouling in the secondary bores. Guess Ill try wiring the secondaries close and see what happens.

What`s wrong is that it`s a vac sec. carb........I don`t like vac. controlling my secondary enrichment ever.

I had a 670 Truck Avenger that had metal left over from machining the fuel blocks in the channels behind the jets (so much for quality control?) I would pull the carb apart and remove all the jets, and then blow out all the channels and air bleeds, and/or run a small wire through them to make sure there is nothing blocking the passages in the carb.
I have had two avenger carbs, and both were tuned pretty lean in the idle and power valve restrictions. I think they were around 0.025" diameter?
I think went to about 0.028 or 0.030" with pin drills and that really helped the idle transition and power under load.

Ill open her up a take a look at the secondary jets and metering block.
I did try wiring the secondaries shut and took her for a test drive. After good off idle accel I hit a flat spot, stumble and then I am able to continue to go up in the rpms, unlike before. Seems better, but not sure what to make of this? I have the stiffest spring(black) installed. Should I go down in secondaies jets?