Engine & Exhaust / How Tos / Tech

Quick Tech: How to Properly Set up the Idle on Holley Carburetors

Holley double pumper carburetors and idle tuning tools

Holley four-barrel carburetors are probably the most common high performance fuel mixing devices on the planet. Given the sheer numbers of carburetors made since the 1950s, you know they work. If there is a fly in the Holley ointment, it’s setting up the idle. Since Holleys are so easy to adjust, it’s also easy to adjust them totally wrong. People will get the idle settings wrong, then blame the carburetor when it doesn’t work properly. That’s a big mistake.

In reality, setting the basic idle on a Holley isn’t difficult. We’ll show you how to do it on both vacuum secondary and double pumper models. What works on this pair of carburetors will work on your high performance Holley, too.

We’ll take you through the entire process in the slide show below:


Tools for Adjusting Carb Idle
Adjusting Idle Mixture Screws
Carb Idle Adjustment
Adjusting Idle Mixture
Four-Corner Idle Circuits
Adjusting Idle Mixture

Here are the tools you’ll need (and it’s not much): a vacuum gauge and a medium size, flat blade screwdriver.

If your car or truck has a camshaft with considerable duration (and as a result, a very lumpy idle), adjusting the idle mixture screws may not help very much. There’s an easy fix: adjust the secondary throttle stop screw. Holley four barrels like this 750 cfm 0-3310C have a small set screw located beside the passenger side throttle bore shaft bore (on the base plate of the carburetor, so you’ll have to flip it over). By turning the screw clockwise approximately 1-1/2 turns, the throttle blade on the secondary will open slightly. This allows a rather large amount of airflow through the carburetor without increasing the “window” seen by the idle transfer slot. At this time, the primary throttle blades can be closed, which tends to enhance the sensitivity of the idle mixture control.

Most carburetors will have the secondary butterflies adjusted with a small length (approximately 0.020-inch, appearing as a square) of the idle transfer slot visible below the bottom edge. If the secondary transfer slots are uncovered excessively as shown here, the engine can develop an undesirable hesitation when the butterflies are suddenly opened. Keep that in mind when adjusting the secondary stop.

With the secondary throttle stop set, the idle mixture screws should be more responsive. Now you can balance the idle mixture. Connect a vacuum gauge to a manifold vacuum source on the engine. Starting with one idle mixture screw, turn it until you get the highest manifold vacuum reading on the gauge. Do the same thing with the other mixture screw. Repeat the process twice more to fine-tune the idle mixture.

If your aftermarket Holley is equipped with four-corner idle circuits like this 850 cfm 0-4781 double pumper, set the front (primary) circuit first, then the rear using the procedure outlined in the previous caption. For the initial adjustment, all idle screws should be within 1/4-turn of each other, although you may have to turn the primary screws out an additional 1/8 to 1/4-turn. Recheck the front idle mixture after the rear setting has been set. Reduce the engine idle speed and repeat the process until the engine idles at the desired rpm. If the carburetor is new or freshly rebuilt, turn the idle mixture screws all the way in, then back them out in unison.

In most cases you’ll find that balancing the idle mixture will increase the idle speed considerably. Reduce the speed to an rpm setting that allows the engine to idle properly. If the vehicle is equipped with an automatic, it should be capable of idling slowly in gear. If you have a large duration camshaft with a lot of overlap, “slow” could mean an idle speed of 1,000 rpm.



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  1. Pingback: Carb People - help needed! - Vintage Mustang Forums

  2. Ernie Muhr says:

    I wish I could read at 500 words a minute. Useless trying to read your information.

    • There’s nothing wrong with the author’s information, it’s perfect in fact.
      YOU need to read each page individually rather than trying to read it while playing the slideshow.

  3. When you said to open the secondary throttle stop 1 1/2 turns that seems like a lot. I did it and the car idled at 2500 rpm and no change when I backed the main idle screw out. Now off with the carb. again tomorrow for a little more trial and error.

    • A lot of racers take a 1/16 drill bit and drill a hole in each primary butterfly for starters. This raises your idle without uncovering too much transfer slot. Remember, when you adjust the secondaries to raise their idle, you are uncovering their transfer slots too. If a 1/16 drill bit is too small you can progress larger.

      • Bill Waters says:

        Only do that “if” you very low idle vacuum under 9 and you have tuned the ifr and or the idle bleeds then if you can get enough idle air then drill the blades

  4. Pingback: Idle Help Please - Page 3 - Chevelle Tech

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  6. Pingback: Adjusting the idle fuel mix screws on a Holley Carb - Vintage Mustang Forums

  7. With a high overlap isn’t a stall converter a remedy for excessive high idle paired to an automatic transmission?

    • Check at idle vac reading, if vac lower than power valve value,carb will put extra fuel in. Causing rough idle or too rich of a idle. if this is the case, put a lower power valve in. Will purr like a cat.

  8. jeff rogers says:

    I’m running two Holley 4160 (450’s) and they don’t have a manifold vacuum port, what else can i do to see if it is running correctly since i can’t hook up a vacuum gauge?

  9. wasteoftime says:

    this site is friken lame its impossible to read it the arthor needs to leave instructions on how to read it ive tried everything somebody please delete this page

  10. Thanks ,it is very good information,,and trully helpfull

  11. Perfect! Just what I was looking for. So simple its dangerous.

  12. If The car has set for five years not running at all, should I rebuild?

  13. I have a big cam in a sb Chevy with a summit electric vacuum pump works great , for booster, only 5lbs of vac at ideal, after warm up not cold ideal goes to 1500 and stays there , I think secondary’s are dropping open because lose of vacuum at ideal what’s the fix for this or do u think it’s something else ???thanks for your time,, Garry !

  14. Dustin Beall says:

    Does this also apply to spreadbore volleys with mechanical secomdaries? Been having trouble adjusting the idle on mine and Cant seem to find the exact info I’m looking for

  15. Scott Baugh says:

    My 4160 Holley does not have the secondary throttle stop screw installed. Do I need? I am running a high duration cam. Honestly don’t recall anymore what the duration is.

  16. Robert Millward says:

    Exactly what I was looking for, thank you for the info!

  17. Mark harris says:

    I have a new Holley and primary circuit will not close enough for proper adj

  18. Buy one of these brackets QFT-BR-67237 from summit racing to adjust the secondary throttle plate. Works like champ. About $20

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