[Jack21]

There are tons of books and references availabe for tuning Holleys that you should be reading, and a Holley website with a lot of information as well.

The Holley 3310 was originally designed for the 396 Chevy big block, and comes out-of-the box with esentially the same jetting. Step one is the idle mixture screws as stated above. Second, take the slotted secondary idle setscrew out and replace it with an Allen head setscrew. Now, you should be able to adjust the secondary idle butterfly adjustment for more secondary air to lean out a too rich primary idle mixture.

You calculate your initial lead by subtracting your centrifugal advance from what your total advance should be, typically 32 to 36 degrees depending on cam and compression. i.e., if you're runing higher compression, say 10:1 (iron heads), 11:1, (aluminum heads), stay closer to 32 degrees total. If lower compression, crank in a few more degrees advance, but no more than 36 deg total. 8 degrees sounds a little low unless your 428 is mid-'60's factory stock.

Leave the option open to scrap the 3310 for a Speed Demon 750 double pumper. Demons provide much better a/f management than Holleys across the board, and the 3310 is the wrong carb for a 428 Cobra. (428 in a 4000# Galaxie with an automatic transmission would be a good choice, just not a Cobra).