Hello this is not a double pumper but a vacuum secondary carb. I know its carb related cause everything else has been checked and set before any carb tuning was done.
I had a 600 quick fuel street slayer carb on this last year and it ran fine out of the box but the idle fuel curve was to rich and could only go 1/2 a turn on the idle mixture screws or it would be pig rich but unfortunately I had an accident with the primary metering block but quick fuel does not sell separate cast metering blocks so its on the side for now but you could smash the throttle or do what ever you wanted to and it ran fine with no backfire off idle just with this holley.
The quick fuel carb had also smaller air bleeds so it had a richer fuel curve and the idle feed restrictions were a massive .035 front and .040 in rear as to where my holley setup was .027 front and .031 rear and that makes a big difference on idle transition af ratio. I opened the holley ifr's on the front to .030 and left the rear block alone and it now has a way better idle and no more surging during low rpm driving but I don't want to go any bigger and mess up my block.
I have done hours and hours on the holley 600 vacuum secondary carb which shares the same metering blocks as the holley 670 street avenger and do a google search and you will see how many people have this problem with all sorts of engine combos and cam sizes. This is what I am stuck with so need to make it work.