The pilot screw on a carburetor sets how much fuel/air mixture goes into the engine. If your engine is running poorly, or if you have a bad idle, you can adjust the pilot screw to get your car running at the highest and smoothest idle. This method will also help to diagnose if your fuel jet is too large or small. If by adjusting the pilot screw you find that the fuel jet is too large or small, you can then fix that problem and have your vehicle purring like a kitten.
Start the vehicle and run it until the temperature gauge shows that it is at normal operating temperature.
Use the throttle stop screw to turn the idle up a few hundred RPM. At higher RPM it will be easier to hear small changes in RPM. Turn the screw 1/4 to 1/2 turn and wait 5 seconds for the carburetor and engine to fully adjust to the changes. Turn the screw in this manner until the idle is 300 to 400 RPM higher.
Turn the pilot screw in until the engine starts to miss and stall.
Turn the screw out. As the screw is turned out the idle should peak and become smooth. Turn the screw past this point until the engine starts to miss and stall again.
Turn the pilot screw back down to the highest and smoothest idle between the two points where the engine missed and stalled. Typical screw settings are between 1 1/2 to 2 1/2 turns. If the optimum idle is somewhere between there it should be good.
Readjust the throttle stop screw back to its appropriate idle speed.
