Been trying to figure out what's wrong with this thing on my own and I'm about to give up:

At the 3/4 throttle spot I'm getting a sputtering, run-out-of-fuel buck. If I back out of it and ease into it I can sometimes get past it. I can downshift and rev it to the moon no problem, it's getting plenty of fuel.

I have changed the cap, rotor, plugs, checked the vacuum lines, checked vacuum advance with a pump and it's not sticking. I routed all the wires so that none are touching each other. Right now I have distributor advance running off the port below the throttle blades.

I changed the fuel filter and checked the needle and seat. It has a Holley 2280 with some 1984 emission stuff on it. Totally convinced that the carb is the problem, I pulled a perfectly working non-emmision 2280 of my '71 and..............no change!

It's most noticible when the engine is cold. While it's warming up you can hear the odd puff in the exhaust note. The fuel mileage is very good but has gone down a touch lately. It fires up with only one pump and after it's warm it starts with just a tap of the key, so I think the timing is ok and the fuel pump is doing it's job.

I am completely out of ideas, so hope you can help.

Thanks.