There are several reasons why your Rochester 4 Jet carburetor may not be idling properly. Here are a few common issues to consider assuming the engine is at operating temperature:
Clogged idle jets: Over time, the small passages in the idle jets can become clogged with debris, preventing the carburetor from delivering the proper amount of fuel at idle. This can cause the engine to stall or run rough at low speeds.
Carefully check each passage in the venturi and the passage coming up from the idle mixture screw. These are very small and easily get plugged. I like to spray air, or spray carb cleaner through each passage to make sure it is clear.
Vacuum leaks: Any leaks in the vacuum system can disrupt the fuel-to-air ratio, leading to rough idling or stalling. Check for cracks or loose connections in the hoses and gaskets.
Incorrect adjustment: The idle mixture screws on the carburetor may need to be adjusted to provide the correct fuel-to-air ratio. This adjustment should be made while the engine is running and warmed up.
The idle may be set below manufactures recommendation. When the idle is too low, the throttle valves are closed too much and will smother the engine.
The main discharge may be leaking. At idle, or just after turning off the engine, look down the carburetor and see if gas is dripping from the discharge. If so, the the main discharge ball may be leaking. Seat the ball by tapping on it lightly. Be sure the spring is above the check ball.
To diagnose the issue with your carburetor, start by checking for any obvious signs of damage or wear. Next, inspect the idle jets and vacuum system for clogs or leaks. If these components appear to be functioning properly, check the idle mixture screws and adjust as necessary. If the issue persists, it may be time to bring your vehicle to a professional mechanic for further diagnosis and repair.