Bigger Carb on my 340

Are you runnning a 1406 or a 1407 on it now?

There is no 650CFM Edelbrock Performer/ AFB.

If you have a 1406, the 340 is pulling through a 600CFM, which is likely too small for that engine with that combination.

Honestly, I think 750 is too much for it. Technically, it will only use as much as it needs, but I think 625-650 would be a good number to shoot for in that engine.

If you don't want to mess around with linkage changing, I would suggest going with one of these, which is an 1807. -

http://www.summitracing.com/parts/edl-1806/overview/

I'm convinced that these designs were all dry CFM rated, which is optimistic, compared to a wet CFM rating.

Either way, I think the AVS, which is essentially a more tunable AFB, will bring you a better run than a 750CFM AFB with that cam, over a 1406 600 CFM AFB, if that is what you are running.

Other companies like Demon and some Holley units are wet CFM rated, which brings true CFM on the advertised rating. I'm running a Street Demon 625, which is likely similar to a thunder series AVS, although I don't really know, because it's a 3BBL, so it's not quite apples to apples.