Stumble off idle -

[decooney]

Grubby,
After checking my notes from my last 428 Cobra with a "mild" cam, and looking back at some threads on this on network54 and mortec comparing the 3310 series to 4779 series 750 carbs it brought back a few memories. Quote "Any way you cut it, the later '750' carbs are lean, especially in a performance application with free-flowing exhaust sytem...much more so with open headers". Just went through this with a friends carb on a 400ci motor and 750 Holley. It took a really good local tuner to help us solve it. Exact same problem. Today's blend of ethanol into fuels is not helping either and having to compensate in some carb circuits is not uncommon these past two years. Kind of a pain really.

If you run out of options and confirm your electrical and ignition system is firing clean, you still have the option of doing what the Holley tec recommended by going up on the primary jetting some. Looking at my notes after lots of testing on my last 428 Cobra with moderate flowing sidepipes, the last jetting I end up with on a standard 3310-2 was 74 primary, 76 secondary with a 6.5 PV. As I recall, there were several of us who used 3310s around here with 74 primary jetting but all later switched to Holley HP or QF-Q carbs for more granular carb circuit tuning. I believe your -11 carb is a little different in overall design and secondary plate as compared to the early 3310s, but the primary circuit is similar enough to compare to for some basic testing. IMO: reading tan plugs gives a generic idea across the board but its hard to diagnose a part-throttle circuit this way without using an AF meter to diagnose it. And, if you don't have a brand new set of plugs and do a clean run and shutoff at a static rpm and using a good targeted beam (medical grade type pen light) light to look down inside the insulator. Even so, its still difficult to troubleshoot a lean-stumble in one circuit this way. The idle circuit can be rich and transition circuit can still be lean. If all else fails and before you give up on that particular carb, you can still try and pull out those 70 primary jets and replace with 74s and a new 6.5 power valve up front (if you have not blocked off the PV). Also, I'm making an assumption you have pulled the entire carb apart and thoroughly cleaned it first. As others pointed out previously, a dirty and gooey carb or poor fitting gasket and vacuum leak will cause the symptoms you are experiencing. If that fails, bring it to a reputable engine tuner with a meter who will find the root cause quickly. Good Luck.