How to Find the Correct Idle Mixture

Engine carburetors control the amount and mixture of gas and air going to the intake manifold. The air-fuel mixture then enters the engine combustion chamber for ignition and combustion. The air-fuel mixture screws on the carburetor control the rich or lean condition of the fuel at idle, remaining constant and not affected by the choke or throttle plates. You must use sequential steps in the proper order to achieve the best lean idle for your engine.

Instructions

1

Place the vehicle in park or neutral, depending upon the transmission type. Apply the emergency brake and raise the hood. Inspect all vacuum lines to the carburetor for tight fit. Remove the air cleaner lid or unsnap the air box lid. Tap the element on the pavement to knock any dust from it. Replace the lid. Check all sensor wires to the carburetor including the electric throttle position and electric choke. The wires should be clean and tight. Start the engine and let it warm up to operating temperature.

2

Make sure the engine runs on all cylinders and the timing was not moved or readjusted. Rev the engine a few times to disengage the chock cam. Locate the idle mixture screws near the base of the carburetor. Identify the idle speed screw above it where it sits on the linkage arms. Refer to your owner's manual for their proper location. Shut the engine off.

3

Hook up one tachometer lead to the positive terminal on the battery, and the other tachometer lead to the negative terminal on the coil or coil pack. If you need to set the base line setting on a rebuilt or new carburetor use a screwdriver to turn the idle speed screw counterclockwise until it disengages from the linkage. Turn the idle speed screw clockwise and when it first engages with the linkage cam turn it an additional one to 1 1/2 turns.

4

Screw the idle mixture screws in until they seat, but do it gently. Turn the idle mixture screws out two full turns, for their base setting. Start the engine and let it completely warm. Recheck the choke to make sure it remains off. Refer to your owner's manual for the correct idle speed in rpms on your engine

5

Look at your tachometer gauge. If the reading on the gauge indicates lower than normal rpm use a screwdriver to turn the idle speed screw clockwise until you arrive at the number. Turn the idle speed screw out if you need to decrease the number. Set the idle speed to manufacture's specifications. If the engine was idling fine and you do not have to set the base line, reserve the idle speed adjustment for last.

6

Turn the mixture clockwise until the rpms on the gauge fall lower and you hear the engine begin to stumble. Use a screwdriver to turn the mixture outward counterclockwise until the rpms fall lower and you hear the engine begin to stumble. Turn the screw back to the highest rpm setting between the two stumbling positions. Fine-tune the adjustment until you reach the highest rpm setting on the gauge. If you have an additional mixture screw adjust it in the same way

7

Check your idle speed reading on the tachometer. It might have changed. Use a screwdriver to turn the idle speed screw inward to increase the rpm to specifications. Turn the screw outward to decrease the rpm setting. Unhook the gauge.