An improperly adjusted electric choke on a Holley four-barrel carburetor can keep your engine from idling properly during cold starts. Starting a cold engine requires a richer air/fuel mixture as cold fuel vaporizes slowly. Therefore, the choke provides a richer mixture by slightly closing off the carburetor’s air supply. When the choke isn’t properly adjusted, it will be difficult to crank over the engine and keep it running long enough for it to heat up to normal operating temperature.
Fortunately, adjusting the electric choke on a Holley four-barrel is simple. There are two different adjustments that can be made:
The choke operation (opening and closing) can be adjusted by rotating the choke cap. There is a center pointer and index marks on the choke mounting (see right) along with an index mark on the thermostatic choke caps. When the vehicle’s ignition is turned on, the heating element inside the choke cap unwinds and allows the choke to open.
You should first check the wire running to the positive terminal of the choke housing. Connect it to a switched source and make sure it shows 12 volts when the ignition is turned on and zero volts when the ignition is off. This test will ensure that there’s proper voltage getting to the heating element, and eliminating this as a cause for poor choke operation.
Before making any changes to the choke setting, you’ll need to allow the engine to completely cool down in between each adjustment. Do not adjust the choke cap beyond the index marks or it will damage the internals of the electric choke unit.
- Loosen the three choke cap locking screws (see right).
- Rotate the choke cap counter-clockwise one notch at a time until the choke performs properly.
If the choke is opening too late:
- Loosen the three choke cap locking screws.
- Rotate the choke cap clockwise one notch at a time until the choke performs properly.
The fast idle cam and screw adjustment control the engine rpm at cold start up. It’s designed to keep the engine idle speed around 1,500 rpm while the choke is engaged. When you start a cold engine and press the accelerator pedal to the floor, the fast idle cam activates and holds the primary throttle plates open a bit more than usual to raise the idle rpm during the warm up. The fast idle cam has a series of steps that will step down (reduce) the idle RPM until the choke plate fully opens.
The fast idle speed adjustment is set at the factory and should give about 1,500-1,600 rpm at cold start up. If the engine is not idling around 1,600 rpm on start up, the fast idle screw will need to be adjusted.
How to Adjust The Fast Idle Speed Screw
- Turn the engine off and let it cool until the choke is fully closed.
- With the engine off, turn the throttle to its side wide open position and hold it there. This will expose the fast idle set screw behind the choke housing.
- Using a ¼ inch open-ended wrench, turn the screw clockwise to increase the rpm or counter clockwise to decrease the rpm (see right).
- Close the throttle and restart the engine and check the rpm.
- Adjust as many times as needed until the engine idles well from a cold start until it is completely warmed up.
- Double check to make sure the idle adjustment screw isn’t rubbing or making contact with the intake manifold just below the carburetor base. Correct this clearance if needed.