Quadrajet, one idle mix screw not working?

Buck Horn Deer Farm said:

My 1977 mercruiser 260 Quadrajet wont idle lower than 800 rpm. Even at 800 it is not a smooth idle. With the throttle linkage removed and idle stop screw backed off, I turn the port side screw in to stop it will stall the motor so I back it out three turns and restart the motor and turn the starbord side screw in to stop or out six turns there is no rpm change at all. I took off the top of the carb and removed the screws and shot wd40 in the screw holes and oil came out the ports in the carb body bore and at the top. Why would the idle screw not change rpms?
When the motor was running I sprayed start fluid around the base and intake manifold and it has no leaks. Whats next?

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