How To Adjust The Idle Mixture On Holley Carbs

By: Dan S | 05/23/2012 < Back to Motor Life Home

Use these simple tips to adjust the idle mixture screws on Holley carburetors to eliminate hesitation, rough idle, backfire, stalling and more...

Holley carburetors have idle mixture screws that control the air/fuel ratio at idle. Improperly set idle mixture screws are often the reason for carburetor symptoms such as hesitation, rough idle, backfire, stalling, hard starting, engine surge, and more. Fortunately, it's easy to adjust the idle mixture with a simple screwdriver and a quality vacuum gauge. The first step is to ensure that there are no vacuum leaks, so check all the hoses leading to and from the carburetor and intake manifold for cracks. 

There are two idle mixture screws on the primary metering block of the carburetor. Some four-barrel models have idle mixture screws on the secondary metering blocks too. In each case you will need to adjust all the idle mixture screws for proper operation. 

Connect a vacuum gauge to a manifold vacuum port and follow these procedures to obtain the highest amount of vacuum reading possible. 

Step 1) With the engine off, start by turning one of the idle mixture screws located on the primary metering block, all the way closed (clockwise), until it bottoms out. The primary metering block is usually the one closest to the front of the engine. 

Step 2) Then turn the same screw counter clockwise one and one half turn. Do the same to all the other idle mixture screws. 

Step 3) Start the engine. If it has a difficult time idling adjust the curb idle speed screw (clockwise) so that the engine won't shut off. 

Step 4) Let the engine warm up and make sure that the carburetor's choke is off. 

Step 5) On a vehicle with an automatic transmission, have someone sit in the car and set the parking brake. Then have them put the transmission in drive while they are depressing the brake pedal. Do not touch or rev the accelerator pedal. On a manual transmission car, the idle speed adjustment can be set with the transmission shifted into neutral and the parking brake is set. 

Step 6) Adjust all of the idle mixture screws by turning them clockwise and counter clockwise, until the vacuum gauge reads the highest possible vacuum reading. All the idle mixture screws should be adjusted the same amount. 

Step 7) Once the highest vacuum reading is achieved, you can adjust the curb idle speed screw so that the engine revs at the proper engine RPMs. 

Once properly adjusted, you'll notice better fuel economy, more power and quicker throttle response. If there's still some hesitation upon acceleration, the next area to check will be the accelerator pump assembly.

Text by: DS Media Relations

A Look Inside Your Holley Carburetor: Tuning Tips- PDF
Carburetor Installation And Tuning DVD

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