One more small 4bbl option is the old Autolite 4100 4bbl. The smaller 1.08 venturi version has a little better manners, but the bigger 1.12 venturi works good too, it is my go-to carb on my 292. And since they are OEM Ford carbs you can generally re-sell them later, especially the 1.08s, the Mustang guys like em.
That said, I wouldn't give up on that 390 Holley yet!
* What list number is it? A 4150 (2 metering blocks) or 4160 (secondary has no block, just a plate). What number is the metering block (just to see if things match up).
* For now, just TRY locking the secondary SHUT, that rules out the possibility it is opening early & causing trouble.
* What jets are in the primary, what number power valve, and how big are the PVCR holes (Power Valve Channel Restriction) the fuel feed holes when you take the power valve out?
* Clean the carb and metering block in brake fluid or diesel, then blend over to gasoline, then over to laquer thinner, then over to acetone, blow dry at each step, kinda like prepping a Super Model's hair, which is how Holley's like to be treated, they are spoiled cuz they're fast and so fussy.
* Check the body-to-baseplate gasket carefully, is it newish & do all the holes line up - the 390 gasket is different so that can cause trouble.
* Make sure the float levels are right up there at the sight plug hole, you've probably done that already.
Well that's a start anyway.
