Dan S

How To Adjust The Idle Mixture On Holley Carbs

Rating: 579 votes, 4.90 average.
Use these simple tips to adjust the idle mixture screws on Holley carburetors to eliminate hesitation, rough idle, backfire, stalling and more...

Click image for larger version. 

Name:	Idle-lead.jpg 
Views:	20387 
Size:	23.7 KB 
ID:	241
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.

Click image for larger version. 

Name:	Idle-1.jpg 
Views:	19699 
Size:	14.0 KB 
ID:	242
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.

Click image for larger version. 

Name:	Idle-2.jpg 
Views:	20655 
Size:	23.0 KB 
ID:	243
Connect a vacuum gauge to a manifold vacuum port and follow these procedures to obtain the highest amount of vacuum reading possible.

Click image for larger version. 

Name:	Idle-4.jpg 
Views:	17512 
Size:	24.0 KB 
ID:	244
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.

Click image for larger version. 

Name:	Idle-5.jpg 
Views:	6112 
Size:	16.9 KB 
ID:	245
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.

Click image for larger version. 

Name:	Idle-8.jpg 
Views:	10928 
Size:	22.7 KB 
ID:	246
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. For more information, vist www.holley.com. You can also watch a video on Holley TV, on how to properly adjust the idle mixture screws.

Text by: DS Media Relations
Photos taken from Holley Carburetor Installation And Tuning DVD

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

Updated 03-27-2013 at 06:13 PM by Dan S

Holley Carburetor Tuning Tips


  1. malcolm2's Avatar
    Looks easy enough.... But I am dealing with a 4160 on a 92 Ski Nautique. I don't have a vacuum port on the carb or the manifold. Do you have a suggestion as to how I will measure the vacuum. The engine is running smoothly, but I do get a rich smell from the exhaust. In this case the boat is being started on dry land, so I don't have to swim around and sniff...
  2. johannes12's Avatar
    My case is a 93 Ski Nautique, and takes forever to wind up and has no holeshot.
  3. jberenyi's Avatar
    I have the same is situation on my Mastercraft. Just disconnect the PCV line and add your vacuum gauge to that port. Works the same way.
About us
Holley has been the undisputed leader in fuel systems for over 100 years. Holley carburetors have powered every NASCAR® Sprint® Cup team and nearly every NHRA® Pro–Stock champion for four decades. Now, Holley EFI is dominating the performance world as well as our products for GM's LS engine. Holley's products also include performance fuel pumps, intake manifolds & engine dress–up products for street performance, race and marine applications. As a single solution, or partnered with products from other Holley companies - Hooker Headers, Flowtech Headers, NOS Nitrous, Weiand, Earl's Performance Plumbing, or Diablosport - Holley products can give you the edge you need over the competition.
Join us