HOW TO REJET HOLLEY CARBURETORS

IDLE SPEED:
1. Set the idle speed using the curb idle speed screw

2. Adjust each idle mixture screw (Figures 3 & 4) 1/8 turn at a time, alternating between each screw. Turn them equally, until you achieve the highest possible vacuum reading without adjusting the curb idle speed screw. Turn screws in to lean the mixture. Turn them out to richen the mixture.

3. Now that the idle mixture is set, it may be necessary to go back and reset the idle speed using the curb idle speed screw.

4. ROUGH IDLE AND VACUUM LEAKS: If a rough idle persists after the engine has been started and the mixture screws adjusted, check for manifold vacuum leaks. These could result from unplugged vacuum fittings or a carburetor flange gasket that was torn during installation. Recheck for proper attachment of all vacuum lines and check the lines for cracks. If the manifold was changed, a manifold vacuum leak could occur at the cylinder head/manifold surface due to damaged gaskets or improper torquing. Frequently, manifold vacuum leaks occur from the valley side of the manifold. These are very difficult to detect, unless a discernible whistle can be heard.

In most cases, when rough idle occurs after a carburetor/manifold change, they result from manifold vacuum leaks similar to those described above. Assuring a proper manifold installation, rather than assuming the carburetor is not functioning properly, will ultimately save time.

FLOAT LEVEL CHECK AND ADJUSTMENT:
Float adjustments are set at the factory, but variations in fuel pressure could cause a change in these settings. The following procedure shows how to make these adjustments:
1. Start the vehicle.
2. Remove the float level sight plug.
3. Observe the sight plug hole for the fuel level. If none is seen, the level is too low. Fuel should be even with the bottom of the sight plug hole. If fuel comes pouring out of the sight plug hole, the float is set too high.
NOTE: A properly set float level will have the fuel level located at the bottom edge of the sight plug hole, as shown by the line in th figure.

4. To adjust, shut down the engine.
5. Loosen the lock screw on top of the fuel bowl just enough to allow you to turn the adjusting nut. Hold the screw in position with the screwdriver.
6. Using a 5/8” wrench, turn the adjusting nut in the appropriate direction: Clockwise to lower the float and counterclockwise to raise the float.
7. Turn the adjusting nut in increments of 1/4 of a rotation.
8. Retighten the lock screw.
9. Restart the vehicle and observe the sight plug hole.
10. Repeat steps 1 through 9, as necessary.

JETTING MAIN JETS:

Due to varied applications that a universal performance carburetor will work with, a few tips on jetting are provided to help you understand their urpose.
1. Out of the box jetting should be close for most applications.
2. Carburetors are calibrated at 70° at sea level. Decrease the jet size, one number for every 2000 ft. increase in altitude.
3. Holley jets are broached, flowed, and stamped according to flow rate. NEVER drill jets, as this seriously alters flow characteristics. Stamped numbers are reference numbers and DO NOT indicate drill size.
4. Spark plugs provide the best indication of proper jetting, see link on READING PLUGS.

 

Holley carbs are calibrated for sea level operation and an inlet air temperature of 70 degrees Fahrenheit. Once you know the correct stock jetting for your particular Holley carb, you can determine whether you live or race at an altitude above sea level. For every 2000 foot increase in altitude, you can reduce the jet size by one size. If you had a carb which has a stock jet size of 80 and you live or race at 2000 feet above sea level, then you would use a #79 Holley jet in the carb. Similarly, a change in the carb's inlet air temperature may require a change in the jet size from the stock calibration. Many racers go a step further by combining all of the weather varibles, temperature, barometric pressure, dewpoint and humidity with the altitude of the track they are racing at to determine the "density altitude". This is a "corrected" altitude above sea level. From there they can determine whether a jet change is necessary to maintain performance or whether to change their "dial in" (if they are bracket drag racers).

 

 

Drag racers should try to optimize jetting by looking for the jet size that gives the best MPH, rather than best elapsed time (ET).

Accelerator pump cams come in various sizes and are color coded and number coded by Holley. The cams have different shaped ramps that the arm from the accelerator pump rides on. By changing the size and shape of the arc on the cam, the pump shot can be tailored to start early or later as you go from off idle to full throttle. Changing the cams can have an effect on the way a vehicle leaves the start line in a drag race. If you leave the line off idle or at a higher RPM (while foot braking or when using a trannie brake or when using a clutch with a manual transmission) experimenting with the pump cams can help. There is no set rule for use, you just have to experiment with the different cams and the different cam positioning holes in the throttle linkage of the carb. Holley sells individual cams or you can buy their kit which includes an assortment of cams to choose from.

Pump shooters are another area of experimentation. Holley carbs come with a standard shooter size which differs by carb list#. If you are experiencing a bog or hesitation off idle, you can try a larger, higher # shooter size. The bog or hesitation may be caused by a momentary lean condition when the carb goes from the idle throttle postion to the main metering system. The shooters help richen this momentary condition and eliminate the stumble. Keep going up in shooter size until a puff of black smoke comes out the exhaust, then go back one or two sizes. Playing with the shooter sizes is particularly helpful, when you have an intake with a large plenum area, such as a large open plenum or a tunnel ram. Keep in mind that as you increase the shooter size, you may also need a "hollow" screw to hold the shooters in the carb. At shooter sizes over .039, Holley recommends that you use the "hollow" screw (PN-26-12) which allows more fuel to flow to the shooters.

POWER VALVES:
The number stamped on a power valve, such as 65, indicates the manifold vacuum below which the power valve is operational. In this case, all manifold vacuums below 6.5” Hg, the power valve is operating. Generally a 65 power valve is sufficient for most high performance applications that have a manifold vacuum of 12” Hg or higher. However, some problems can result with radically-cammed machines equipped with automatic transmissions. These vehicles often “idle” at 2000 rpm, approx. 6.0” Hg. At this point the main nozzles are starting to feed and richen the mixture (supplied by the power valve) and the engine will probably “load up”. To correct this problem, install a 45 or 35 power valve. If the engine has a manifold vacuum of 12” Hg or less, a good way to determine power valve size is take the manifold vacuum at idle and divide that number by two. The answer is the power valve size. This will provide idling and proper fuel flow under wide-open throttle conditions when manifold vacuums seldom rise above 1” Hg.

POWER VALVES.

There is a lot of misunderstanding concerning power valves in Holley carbs. Many 4-barrels come with a particular power valve depending on the carb list# and application. Some carbs have two power valves, while others only have one. The power valves are numbered by the amount of engine vacuum in inches at which they will open and add additional fuel to the power circuit. In other words a 6.5 power valve will open when the vacuum signal on the engine drops below 6.5" and will remain closed above that amount. One of the misconceptions is that they can't be trusted to work because an engine backfire or "belch" can "blow out" the power valve. Many Holley performance carbs models and list#'s now come with built in power valve "blow out" protection which eliminates this problem. If you have an older model carb you can purchase a small, inexpensive, easy to install kit from Holley (PN - 125-500) that will also protect the power valves in case of an engine blowback thru the carb. CENTEK in Redmond, Oregon, (see their website at www.powervalveshield.com ) also sells an inexpensive Holley power valve blow out protector, "Power Valve Shield", which takes about two minutes to install and does not require any drilling.

Many tuners will automatically remove the power valves and use a "plug" thinking this is the "hot" ticket. However, if the power valve is removed and plugged, the main jet size must be increased 6-10 jet sizes to make up the required fuel amount lost by the removal of the power valve. In addition, when the power valve circuit is plugged, the part throttle fuel economy is worsened and may become overly rich. Plug fouling may become a problem at part throttle.

Stock engines can have high vacuum readings (10-18 inches at idle) and the Holley power valves with higher readings like 6.5 to 10.5 will work correctly. Long duration non-stock camshafts and other performance related parts can cause a problem, because engine manifold vacuum may be lowered with these performance parts and the power valve, if incorrect, will always be open, even at part throttle, leading to an overly rich air/fuel mixture. The solution is to choose the correct power valve and to do that you need a vacuum gauge. On a manual transmission vehicle, hook up the vacuum gauge and take the reading with the engine at idle. Then use a power valve that is rated 1-2 inches below that amount. For example, a motor that shows 7" of vacuum at idle should use a 6.5 or 5.5 rated power valve. If you have an automatic transmission, take the vacuum reading at idle in "Drive" (with the emergency brake on and the wheels blocked) and chose the power valve 1-2 inches below that figure. You can get a little more detailed information by driving the car with a vacuum gauge hooked up with a longer hose so you can read it while driving. Drive the car at medium loads and while cruising and note the various vacuum readings. Then chose the appropriate power valve rating.

Holley makes performance style "standard" flow or a "high" flow power valve which has a large opening. "Single stage" power valves are available in 1" increment sizes from 2.5" thru 10.5". There are "two stage" power valves available that are more for "economy" minded users rather than "performance" enthusiasts.

VACUUM SECONDARIES:

There are a number of ways to tune the moment when the vacuum secondaries open on a Holley four barrel carb. The vacuum secondaries are controlled by a diaphram and a color coded spring. Holley makes a number of different springs with different tension on the springs. You can change the springs and change the opening moment. The color coded springs run from light tension to heavy:

White - Lightest
Yellow (Short Spring)  
Yellow  
Purple  
Plain (Steel grey)  
Brown  
Black - Heaviest

If there is a bog or hesitation when the secondaries open, the spring tension is too light, go to the next heavier spring. Holley offers a kit (PN-20-13) which contains one each of the above color springs. When you change springs you'll note that the stock cover over the spring and diaphram is not all that easy to get to. Holley makes a special cover (PN-20-59) that makes spring changes quick. Finally, Holley also makes a completely adjustable thumbscrew operated diaphram cover (PN-20-99). This cover limits the travel of the diaphram and therefore limits how far the secondary throttle plate can open. It makes secondary throttle opening adjustable. It's very nice to have if you are a bracket racer and are using a carb with vacuum secondaries. You can adjust the throttle for changes in weather and track conditions or for changes in your "dial in".