Holley carburators are simple machines, but if they're not Tuned properly, they won't deliver all the performance they're capable of. We hooked up with Jeff Schwartz for a few tricks he uses to make sure there are no stumbles, bogs, or hiccups related to the tune on a Holley Performance Products 850 CFM Double Pumper carburetor. Click Read More for photos and the video!
The first step to a properly adjusted is to make sure there is no slop in the accelerator pump linkage. The accelerator pump (or pumps, in case of double-pumpers) can be found on the throttle-plate side below the float bowl. This photo shows where the throttle linkage meets the lever on the accelerator pump.
If your linkage has any play between the arm and the pump lever, you can adjust the length of the pushrod with a pair of wrenches.
Once the linkage is tight, the next step is to set the idle using a vacuum gauge. When the engine is making the highest vacuum amount at idle, it's going to idle at its optimal RPM.
Connect the vacuum gauge to a manifold vacuum port like the one shown. Most Holleys have mulitple ports, use the manifold vacuum connection below the throttle plates. This way, your vacuum gauge measures engine vacuum with the throttle plates closed.
You can adjust the idle with the idle adjustment screws. This is done with the vehicle running. Some models have multiple adjustments, turn them all the way in and then back them out 1/2 turn at a time until the vacuum gauge reads the highest amount.
Inside the float bowls live the floats. Makes sense, huh? Once the idle is set, Schwartz recommends setting the float level.
This shot shows the float bowl with the sight plug removed. The fuel level inside the float bowl is at its proper level right below the sight glass port. This should be checked on the vehicle. Opening the sight plugs will probably spill fuel onto the engine - be sure to have a Fire Extinguisher close and make sure you clean up any spilled fuel. Holley makes clear sight plugs to make this job easier... they're made of plastic and should be used for checking the levels only. You should then reinstall the brass ones so the clear ones do not Haze over from the gasoline dissolving them.
To set the float level, loosen the retaining nut and turn the screw head. When the level is correct, retighten the locking nut.
Once you set the linkage, idle, and float levels, you can address any hesitation issues you may have, either off-idle or part throttle. This photo shows an accelerator pump discharge nozzle. These are marked with a number, which corresponds to the size of the opening in thousadths of an inch. For example, a 35 nozzle has an opening size of .035".
Here's the location of the primary accelerator pump discharge nozzles. These are easily changed with a screwdriver.
The general rule is that you increase the size of the primary discharge nozzles if you have an off-idle stumble, and you increase the secondary discharge nozzles if you have a part-throttle hesitation.
Here's where the secondary discharge nozzles live in the main carb body.
If you find you still need a bigger shot of fuel, you can increase the squirt of your acceleator pump by installing a more aggressive accelerator pump cam. You can see the difference in the three shown - Holley makes a variety of these in different colors.
Here's where the discharge cam lives on the throttle linkage.
Next up is proper power valve selection. All Holley power valves are designed to enrich the mixture under acceleration. They have a number stamped in the case; this number corresponds to the vacuum rating when the valve will open. For example, the 4 valve shown opens at 4 inches of vacuum. The proper selection of your power valve should be done with a vacuum gauge. Connect the gauge, fire up the car, and take the vacuum reading with the car idling in gear. The power valve should have a rating 1 inch below this vacuum measurement.
Here's the power valve installed in the main body. Also visible here are the main jets.
If you follow these steps, you'll be on your way to a properly dialed-in Holley carb. Of course, you still need to make sure the carb is matched to the engine requirements, and that your ignition and fuel systems are all up to the task. At any rate, you'll find more info on Holley tuning at the Holley website.
Holley Performance Products
Schwartz Extreme Performance
Schwartz Extreme Performance
6205 Lou Avenue
Crystal Lake, IL 60014
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