How To Tune Most Holley Carbs, Part 1

Do all adjustments outdoors, on level ground, away from structures, and keep a fire extungusher handy.

Carb tuners live and die by the vacuum gauge,
So get a good one that has a large, easy to read face.

(Sears Craftsman model 161.217901, or better, pressure & vacuum gauge is what I recommend.)

More On Tuning & Trouble Shooting With A Vacuum Gauge.

Don't get one of the 'no name' plastic cheapies. You will only hurt your self in the long run.
Get one that does vacuum and pressure, so you can check fuel pressure and exhaust back pressure with it also.
If you have to buy one, might as well make it do double duty, and get twice your money's worth.

The Steps To Tuning A Holley Carburetor;

Do These adjustments while the vehicle is NOT RUNNING.
Do these adjustments in the order they are listed here, because quite often, one adjustment will affect another.

  • Throttle Blade Position- Throttle Opening. (The most common mistake)
    You will need an extra person for this test.
    Have someone hold the gas pedal all the way to the floor, and look down the throat of the carb.
    If the throttle blades are not coming exactly perpendicular, then adjust the linkage until they do.
    (Usually by moving the connection point closer or farther away from the primary throttle shaft)
    Most of the time the blades aren't opening all of the way.
    Adjust the mounting hardware until the blades open all the way, without going over center.
  • Adjusting The Accelerator Pump Linkage.
    You will need three hands for this.
    With the pedal all the way to the floor, (or the throttle linkage held all the way open),
    use a 1/4" wrench to hold the accelerator pump lever all the way down, (the lever attached to the lower right corner of the float bowl, DO NOT BEND THIS ARM)

    Adjust the spring and bolt assembly that contacts it until you can slip a 0.015" feeler gauge in between the spring bolt and the accelerator pump arm with the arm held fully down.
    If you have a double pumper, you have to do this on both ends.
    (If you notice, inside the throttle shaft linkage there are plastic cams that operate the accelerator pump linkage. These cams can be changed to alter the amount and timing of the accelerator pump shot when you get good at this stuff)

  • Idle Mixture Adjustment.
    The Metering Block is the 3/4" thick metal component between the float bowl, and the Venturi body.
    There is an adjustment 'screw' in both sides of your front metering block.
    This screw should VERY GENTLY be turned in all the way until it bottoms out, and backed off 1-1/4 turns on both sides. This will be your idle adjustment starting point.
    Fine tune is done with the engine running, so see below for final adjustment instructions.

    BE VERY CAREFUL! These screws are very easy to twist the noses off of in the idle passages, so very gently bottom them out, and immediately back them out.
    Unless you have a secondary metering block, and mixture screws in the secondary metering block, you are done with this for now.
    (If you have a carb that has a foil tag over the idle adjustment screw that says 'LEAN' and an arrow pointing counter-clockwise, disregard the above instruction)

  • Air Cleaner. (Third most common mistake)
    This may look out of place here, but it is not.
    Too small of an air cleaner can create too much of a low pressure area at the top of the carb.
    This will affect the air bleeds that dictate fuel mixtures, and it will affect float level.
    The rule of thumb is, use the largest & tallest air cleaner you can fit under the hood. One of the 'Dished' type air filter cases that the bottom fits down over the carb is strongly recommended.
    Holley, Mr. Gasket, Edelbrock, and a hundred others sell them.

    Us as much filter as you can. The less you torture the incoming air, the more power your engine will put out.
    Stacking short air cleaners for height is OK, but small diameter air cleaners are not.
    The average 350 CID engine requires a 4" tall X 14" diameter air cleaner to operate correctly.
    (Alternatively, a 4.5" tall X 12" diameter, 6" tall X 9" diameter will work)
    All parts of what ever air cleaner you use should be kept at least 3/4" from the float bowl vents of a Holley carb.
    (Float bowl vent, that little 'pipe' that sticks up just inside the air cleaner rim on the Venturi body)

  • Ported Spark Vacuum. (The 4th biggest mistake made)
    There is a vacuum port on the upper left side (as you stand in front of the vehicle looking in the engine bay) of the primary metering block.
    This is ported spark vacuum.
    It is drawn from the Venturi bore, not the intake manifold or carb base plate.
    This is where your vacuum advance for the distributor should be attached.

  • Vacuum Leaks.
    This is a real problem.
    Vacuum leaks occur when the carb isn't tightened down enough, or is tightened down too much and warps the baseplate.
    They occur when vacuum ports on the carb are not properly plugged.
    Vacuum leaks occur in the intake manifold when ports are not properly plugged or fittings are leaking.
    Take pains to make sure ALL sources of vacuum leaks are plugged, or properly routed to appliances that don't leak.



    This will greatly reduce the possibility of fire while making adjustments.

    If you have a mechanical fuel pump,
    You have no choice but to start the vehicle.

    If you have custom paint on the carb or intake, you may want to take steps to keep the fuel off of it, or be ready to wash it down right after making the float adjustment.

  • Float Level. (Second biggest screw-up)
    There is a Sight Plug in the side of a Holley float bowl. It will be a brass or black 'Screw'.
    Take it out.
    Fuel should just be barely be seeping out. If not, ...

    There is a nut, with a screw in the top of it, very near the fuel line inlet.
    The screw is the lock, the nut is the fuel level adjustment.
    Loosen (Don't take out) the screw about 1 to 1-1/2 turns.
    (If you remove the screw, fuel will gush out the screw hole.)

    Turn the NUT clockwise (IN) to lower the fuel level.
    Turn the NUT counter-clockwise (OUT) to lower your fuel level.
    Hold the nut while you tighten the screw up to keep your adjustment.
    (This must be done on both ends of every 4 barrel Holley.)


  • Idle Mixture Adjustment.
    Start vehicle, and let it warm up.
    Connect a vacuum gauge to an full time vacuum source (intake manifold).
    The carburetor throttle plate is just fine. (Very bottom part of the carb)
    With the vehicle running, (and in gear with an automatic) Turn the idle mixture screws on the metering block clockwise the same amount until you get your highest vacuum reading.
    Start with about 1/4 turn at time, and KEEP BOTH SIDES THE SAME!
    If, on rare occasion, the vacuum reading goes down when the screws go in, by all means turn them out 1/4 turn at a time until the highest vacuum reading is achieved.
    Don't forget to re-set the curb idle speed when you are done with the idle mixture.

  • Idle Speed/ Curb Idle Speed.
    Idle Speed and Curb Idle Speed are not the same thing.
    Idle speed is the RPM the engine turns when there is no load.
    (Like when out of gear, or the clutch is in)
    Curb Idle Speed is when the automatic transmission is in gear (emergency brake set, and wheels chalked please), and the alternator is pulling a load.
    The idle adjustment is usually a screw with a spring around it, somewhere very near the primary throttle shaft on the drivers side.
    Turn the screw clockwise (IN) to increase the RPM.
    Turn the screw counter-clockwise (OUT) to decrease the RPM.
    The average Holley should have the CURB IDLE SPEED set at about 600 RPM for automatic transmissions, and only go up about 200 RPM when out of gear.
    Manual transmissions should be set at about 700 RPM all the time.

    All content and web site screw-ups are entirely mine!