1969 Cutlass S 350. Holley 650 4BBL carb. Turbo 350 tranny.

Engine and tranny were complete rebuilds , Holley 650 is new.

The engine idles fine but idle speed drops when put in gear. Especially after it's warmed up. Idle speed is around 850, drops well below 500 when in gear and brake pedal depressed.

Any ideas on how to fix this?

 

Start with basic tuneup.
Ensure timing is adequate.
Tune the carburetor.