Holley running rich at part throttle cruise

Change the primary air bleeds go 2 sizes bigger or drill them if yours are not threaded.. Or you can pull yours and thread the holes , there is a kit for that. Also Drill 3/32 holes in all 4 of the venturi valves. This will make a big difference if you don't have a 4 corner idle circuit carburetor. You probably are out of square on your transfer slots.

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Carb is 750hp with four corner idle and adj air bleeds.

I will double check the transition slots they were set years ago but may have changed fiddling with idle speed.