Could it be THIS simple?

[HeadDoctor]

Brian
It was just a thought I didn't know you still drove it on the street - yes then it is best to change over to the 4-corner and 2 power valves the engine will run a bunch cleaner. When I convert Holleys to 4-corner I always use 3310 metering plates. I don't have the part # handy but its in their parts book.

That overly rich condition is common for Holleys

The main jet has nothing to do with the idle circuit and only partially controls the intermediate transfer slot just above the the idle hole in the base-plate.

I'm assuming that you have a 4779-2 750
It doesn't have holes drilled in the butterflyes and to get it to idle you have to turn the RPM screw about 3 to 5 turns to get idle speed. If that is correct then with the blades open that far the boosters are trying to dribble causeing the rich idle.

To find out - hold the throttle lever tight against its stop (you might want to have someone help you with this) and unscrew the adjustment until it is completely closed - then turn the screw in 2 turns - Start the car and see if it will idle there out of gear - if not then you are going to have to drill holes in the butterflyes to conpensate for the added air/fuel - Of course this may not be the problem at all! It could be something internal ????

At this point all we can do is make suggestions

Denny@JDS Induction Products