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To preserve warranty, these instructions must be read and followed thoroughly before and during installation.ย
NOTE: The tuning instructions are intended to supplement the rebuilding instructions provided with this kit. It is important that the carburetor be rebuilt and adjusted to the proper specifications before any performance tuning is attempted on the carburetor.
NOTE: Some performance modifications (particularly those which affect primary throttle calibration) may alter the exhaust emissions performance of the vehicle and should be considered for off-road use only.
NOTE: Two barrel carburetors should be โtunedโ in the same manner as the primary side of a four barrel carburetor.
NOTE: Reassemble carburetor to stock form, so fine tuning efforts can be analyzed with respect to the original calibration.
Before you begin to tune your carburetor for your particular vehicle, you must get a โfeelโ for your vehicles performance so that any changes you make (good or bad) will be readily apparent. Be patient and make only one change at a time so only that change can be fully analyzed. This cannot be over emphasized as there are no โshort-cutsโ to peak performance. Recording each change and the resulting performance increase or decrease will provide you with a handbook of how vehicle performance is affected by individual carburetor adjustments. This may be helpful in the future or on other applications.
Idle mixture needles control the air/fuel mixture at idle. When tuning the idle mixture, youโre actually tuning for the best manifold vacuum. Idle mixture needles are found on the primary metering blocks. If you change one idle mixture needle, you must change the other idle mixture needle by the same amount. Here are the proper steps for setting the idle mixture needles.
The accelerator pump system consists of three main components: the pump diaphragm, the pump cam and the pump nozzle. This is the carburetor system that is most responsible for having good, crisp, off-idle throttle response. Its purpose is to inject a certain amount of fuel down the throttle bores when the throttle is opened. By accomplishing this purpose it acts to smooth the transition between the idle and main circuits so that no stumble, hesitation or sluggishness will be evident during this transition phase
NOTE: The old saying โif a little is good, a lot is betterโ does not apply to the proper tuning of the accelerator pump. Your carโs performance can be just as bad if it receives too much fuel as if it receives too little fuel.
Two factors affect the accelerator pumpโs delivery: First, The amount of fuel that can be delivered by one accelerator pump stroke. This is determined by the pumpโs capacity and the profile of the pump cam. Secondly, the period of time that it will take for this pre-determined amount of fuel to be delivered. This is affected by the pump nozzle size.
A larger pump nozzle will allow this fuel to be delivered much sooner than a smaller pump nozzle. If you need more pump shot sooner, then a larger pump nozzle size is required. During acceleration tests, if you notice that the car first hesitates and then picks up, itโs a sure bet that the pump nozzle size should be increased. A backfire (lean condition) on acceleration also calls for a step up in pump nozzle size. Conversely, if off-idle acceleration does not feel crisp or clean, then the pump nozzle size may already be too large. In this case a smaller size is required.
Holley accelerator pump nozzles are stamped with a number (Figure 5) which indicates the drilled pump hole size. For example, a pump nozzle stamped โ35โ is drilled .035". Pump nozzle sizes are available from .025" to .052". Please note that whenever a .040" or larger accelerator pump nozzle is installed the โhollowโ pump nozzle screw should also be used. This screw will allow more fuel to flow to the pump nozzle, assuring that the pump nozzle itself will be the limiting restriction in the accelerator pump fuel supply system.
NOTE: : When changing the pump nozzle itโs best to jump three sizes. For example, if there is currently an off-line hesitation with #28 (.028โ) pump nozzle, try a #31 (.031โ) pump nozzle.
Once a pump nozzle size selection has been made the accelerator pump system can be further tailored with the pump cam. Holley offers an assortment of different pump cams, each with uniquely different lift and duration profiles that are available under Holley P/N 20-12. Switching cams will directly affect the movement of the accelerator pump lever and, subsequently, the amount of fuel available at the pump nozzle. Lay out the pump cams side by side and note the profile differences. This little exercise may help to better explain the differences between the cams and their effect on pump action.
Installing a pump cam is straightforward (Figure 6). Itโs a simple matter of loosening one screw, placing the new pump cam next to the throttle lever and tightening it up. There are two and sometimes three holes in each pump cam, numbered 1, 2 and 3. Placing the screw in position #1 activates the accelerator pump a little early, allowing full use of the pumpโs capacity. Generally, vehicles which normally run at lower idle speeds (600 or 700 RPM) find this position more useful because they can have a good pump shot available coming right off this relatively low idle. Positions #2 and #3 delay the pump action, relatively speaking. These two cam positions are good for engines that idle around 1000 RPM and above. Repositioning the cam in this way makes allowance for the extra throttle rotation required to maintain the relatively higher idle setting. Pump arm adjustment and clearance should be checked and verified each and every time the pump cam and/or pump cam position is changed.
Many people have the misconception that opening the secondary throttles sooner will provide increased performance and quicker drag strip times. Others think they must โfeelโ a kick when the secondaries engage. Still others believe that they should disconnect the vacuum diaphragm and make the secondaries open mechanically.
Before going any further, letโs discuss these points in a reverse order. First, if we could make our vacuum operated secondary carburetors perform better by opening the secondaries mechanically, it would be to our advantage to do so since all that vacuum actuating hardware is expensive and requires much time and money to calibrate. Mechanical secondary carburetors all utilize a secondary pump shot to prevent bogging when the secondaries are opened. Secondly, those who โfeelโ a kick when the secondaries engage are actually feeling a flat spot during initial acceleration, because the secondaries have already begun to open and have weakened the fuel delivery signal to the primary boosters. The engine is struggling to increase speed and what they actually feel are the secondary nozzles โcrashing inโ as the engine finally reaches the speed where it provides the proper fuel delivery signal to primary and secondary venturi. Third, opening the secondaries early causes the situation described above. The secondaries must not open until the engine requires the additional air. This allows torque to increase along the peak torque curve. Performance is compromised less by holding the secondaries closed a little longer than by opening them a little too soon. If the opening rate of the vacuum operated secondaries is properly calibrated there should not be a โkickโ, only a smooth increase in power should be felt.
This kit contains three secondary diaphragm springs. The larger diameter of the wire the stiffer the spring and the later the secondaries will open.
Secondary diaphragm springs are replaced as follows:
TIP:To make reassembling the diaphragm easier, extend the diaphragm into the housing and form the diaphragm into a โmushroomโ shape (Figure 11). Then position the diaphragm over the bolt holes and vacuum port. Hold the diaphragm in that position with your fingers while inserting the spring, cover, and bolts to avoid disturbing the position of the diaphragm.
In general, heavier cars require stiffer secondary diaphragm springs than light cars. Air cleaner configuration and restriction plays an important part in spring selection also, so be sure to use your air cleaner when evaluating your vehicles performance after each change. DO NOT expect to โwingโ the throttle and see the secondaries begin to open. If they do they will probably open too soon. Secondaries should open only when the engine is under a load. DO NOT clip or trim a spring in an effort to make a spring lighter so that the secondaries will open sooner. Strange as it may seem, clipping springs actually increases spring rate and will delay opening. If additional springs are required, they may be purchased from your local Holley retailer (Holley P/N 20-13).
Due to varied applications that a universal performance carburetor will work with, no jets have been included in this kit; however a few tips on jetting are provided to help you understand their purpose.
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 6.5โ Hg, the power valve is operating. Generally a 65 power valve is sufficient for most high performance applications that have a manifold vacuum of 12โ Hg or higher. However, some problems can result with radically cammed engines equipped with automatic transmissions. These vehicles often โidleโ at 2000 rpm, approx. 6.0โ 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 โload upโ. To correct this problem, install a 45 or 35 power valve. If the engine has a manifold vacuum of 12โ 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 1โ Hg.
Experimenting with air bleeds is not recommended. Countless hours of testing on expensive flow stands is required to obtain the proper bleed sizes for a given calibration. It is unlikely that a better bleed combination can be obtained without extensive experience and facilities.
Jet extension (Holley P/N 122-5000) can be used in conjunction with the secondary main jets to prevent the jets from being uncovered as fuel rushes to the back of the fuel bowl during extreme acceleration. Jet extensions require the use of a notched float. If you do not have a notched float, a notched float with jet extension is available as Holley P/N 116-10
For some strip use it is recommended to discard the moraine fuel filters which are sometimes installed behind the fuel inlet. If this is done install an inline Holley fuel filter P/N 162-523, between the fuel pump and the carburetor.
7-1/2 psi maximum
5 psi minimum
NOTE: Fuel pressures above 7-1/2 psi can create severe fuel control problems and are not recommended.