All good advice so far.

You say the carb has low hours on it. Has it always idled rich (we're assuming), or is this a new occurrence?

Be sure the idle air bleeds are not clogged. Look down into the top of the carb, into the venturis. On the primary side, you will see 4 little brass orfices that are pressed in. The outboards are the IABs. Run some very fine small diameter solid wire down each one, and follow it with a good blast of carb spray cleaner. Preferably, this will be done with the float bowls and metering blocks off so any dislodged crud will be washed out. Do the same proceedure on the secondary side. Blocked IABs will cause a rich idle.

While the metering blocks are off, take a large good quality file and carefully file the surface of the main body where it mates with the metering blocks. Rarely is this surface flat, and if it's bad enough, you can get internal leaks because the metering blocks are not seated tightly against the main body.

Put it back together using only genuine Holley blue reusable metering block gaskets. No cheap cork or (shudder) rubber gaskets.

If you have an electric fuel pump laying around, pump the carb full of gasoline and let it sit on the workbench for a day or two. If the power
valve(s) (I think your 770 has only a PV in the primary side) have a ruptured diaphragm(s) as suggested above, gasoline will leak out through the rupture and out onto the bench. If you don't have a pump, put the carb on the car, start the engine (to pump it full) the renove the carb and leave it on the bench for a while to observe for leaks. Find 4 long bolts and 8 nuts and put the carb "on stilts" for easy observation.

ETA: I missed this the first time:

"I can adjust the mix screws with little afect.."

Flip the carb over and look at the relationship between the throttle plates and the transfer slots at idle. The throttle plates should expose enough of the transfer slots at idle that all you see is a square. See photo. If more transfer slot is exposed, that could cause your problem. Shout back if this is the case.


Jim