But I'm leaning towards ditching the 770 Street Avenger for a 750 3310 Vacuum Secondary Carb.
I'm on my second 3310 vacuum secondary carb on the 383 and having very similar issues with idle. I'm wishing I had a QuickFuel 750 like is on the 521 Lincoln. Not wishing you any bad luck but I'm feeling a little less incompetent when I hear that a pro with a dyno didn't solve it quickly.
The QF has replaceable idle and main air bleeds and replaceable power valve restrictions. The 521 is running SWEET after we worked on all that stuff. Had it out a couple of days ago and enjoyed driving it a lot
If I had the QF, I'd be replacing the idle air bleeds.
Doesn't it make you want an aftermarket EFI? If I were looking at buying another carb, I'd also be looking at EFI.