Olds 455 idle issue

This is a 1976 olds on which I put new heads last Fall; freshwater keel-cooled block. Decided to give it new points, plugs, plug wires and a rebuilt carburetor while at it. It is a Rochester 4 brl with manual choke (heated by the crossover intake).

On startup, the engine runs beautifully. No hesitation when given throttle and idles at 800 smooth as can be. When using the boat, she runs well and strong at operating speed (2000-2500rpm) but I am having an idle issue once the engine reaches full operating temperature. Engine is missing or starving and will cut out if I don't work to keep the rpm at 1000+. The problem is unnoticable if returned to operating speed.

What are some potential causes and trouble shooting for this difference in cold versus hot performance? Thanks.

 

The answer is in the question --check your choke would be my first bet.

 

I've adjusted the choke simply by turning it to compress the spring, just closing the butterfly while cold. Any suggestions for how to fine tune? If the butterfly is not set the way i have done it, the engine will not start when cold. Maybe i will abandon the choke and jack the plate with a screw driver to start it and see how it runs when hot.

 

Sounds like an air leak on the intake , or perhaps vapor lock from the modern crap fuel.

Check the carb mount gasket .

Check the fuel pipe is not too close to the exhaust.