360 Rebuild for 1974 Swinger

Bl1zzard said

Been tuning it up and fixing leaks, etc. Went for a couple of boots down the back roads, nothing over 55mph or 3000RPM lots of RPM changes.

I've never had a Holley 750 DP to tune before, but it seems weird. It has 4 idle mixture screws. Best idle with maximum vacuum has the front 2 screws turned almost all the way in, maybe a 1/16th of a turn out. The back 2 screws are ~1 turn each. Seems a little counterintuitive, but that where the idle is highest and vacuum is 15 and steady....

Click to expand...

That doesn't sound right. I'm no carb expert, and never had a double pumper either, but you're basically idling on the secondary circuits, which sounds like you have an internal leak or something out of whack. Was it rebuilt, or is it new, or...? You should be able to get them somewhat balanced.

Check this:

"If your car or truck has a camshaft with considerable duration (and as a result, a very lumpy idle), adjusting the idle mixture screws may not help very much. There’s an easy fix: adjust the secondary throttle stop screw. Holley four barrels like this 750 cfm 0-3310C have a small set screw located beside the passenger side throttle bore shaft bore (on the base plate of the carburetor, so you’ll have to flip it over). By turning the screw clockwise approximately 1-1/2 turns, the throttle blade on the secondary will open slightly. This allows a rather large amount of airflow through the carburetor without increasing the “window” seen by the idle transfer slot. At this time, the primary throttle blades can be closed, which tends to enhance the sensitivity of the idle mixture control."

from Quick Tech: How to Properly Set up the Idle on Holley Carburetors - OnAllCylinders