Q: How do you tune a vacuum-operated secondary?
Holley Performance responds: Carburetors withvacuum-operated secondaries are popular for street vehicles because theyare very forgiving, and correct carburetor size selection is not ascritical as with mechanical-secondary carburetors. The vacuumsecondaries only allow the carburetor to flow the necessary air and fuelbased on the engine's demand. Vacuum for the diaphragm is obtained fromthe primary venturis with some being bled off through an opening intoone of the secondaries. As engine rpm goes up, velocity through theprimaries creates a vacuum signal. The amount of secondary opening isinitially dependent on airflow in the primaries but is later augmentedby the vacuum in the secondary barrel through the previously mentionedbleed opening. If a carburetor is too large for an engine, the vacuumdiaphragm "sizes" the carburetor so it flows only the necessary amountby partially opening the secondaries.
Tuning the vacuum secondaries is very simple. Holley offers color-codedsprings of various tensions. This enables the engine's vacuum to pullopen the secondaries either sooner or later depending on the springtension. White springs are the lightest and allow the secondaries toopen sooner, while black Holley springs delay secondary opening thelongest and open them at the slowest rate. Holley spring kit PN 20-13includes seven springs for proper tuning.
Q: How do you set the float level and what effect does ithave?
Demon Carbs responds: For street-driven Demoncarburetor applications, we recommend starting with the float level 1/4up the sight window. Changing the float level changes the amount of fuelin the bowl (reservoir) to feed to carburetor jets. Raising the floatlevel can help the engine respond quicker if you're having a leanstumble, while lowering the float level can delay the start of the mainmetering circuit, in effect helping lean the mixture coming off idle.
Q: We've seen some carburetors with drilled throttlebutterflies. Why and when is it necessary?
Holley Performance responds: Yes, some Holley carbs docome with drilled butterflies. Typically these are found on ourHP-series carbs. This was a performance feature that we added becausethese carburetors are mostly purchased for high-performance race enginesthat require extra air when the butterflies are closed to idle properly.Sometimes drilling the butterflies is required on carburetors forengines utilizing a cam with a narrow lobe separation, which in turncreates low manifold vacuum. The large cam requires more air and fuel toidle. Most people open the curb idle screw so much that the butterfliesopen far enough to expose the main circuits, causing engine rpm to jumpso high that turning the idle-mixture screws have no effect on enginerpm. Usually this condition can be corrected by closing the curb idlescrew down to just below the idle transfer circuit slot and opening upthe secondary throttle blades with the secondary-throttle-blade screw.Many people are not aware that there is a secondary-throttle-openingscrew. In rare occasions, drilling the butterflies would still berequired if opening up both sets of throttle blades did not provideenough air for the engine.
Q: When should I use a double-pumper instead of avacuum-secondary carb?
Demon Carbs responds: Generally you should use amechanical-secondary carburetor on vehicles equipped with manualtransmissions or with an automatic transmission using a3,000-rpm-or-higher stall converter. On automatic-transmission-equippedvehicles with less than 3,000-stall converters, the vacuum-secondarycarburetor is the best choice because the design of the double-pumpercan dump so much extra fuel into the engine that a rich bog may occur.
Q: How do you diagnose the cause of a bog or stumble?
Edelbrock responds: There are different kinds of "bogs"and "stumbles"; each situation has a different meaning to differentpeople, and a different feel as well. You can have a lean bog or a richbog; you can have the same with a stumble. Keep in mind that a bog and astumble are pretty much the same thing. If the vehicle tends to fall onits face--step on the throttle and the engine just wants todie--typically this is a rich bog. If too much fuel enters the enginetoo soon and the engine can't handle it, it makes no power and doesn'twant to go anywhere. A lean bog or stumble would give more of a jerkymotion and tend to "pop" back through the carburetor. No matter whatsituation you encounter, you will need to check the basics first,including fuel pressure, float level, ignition timing, and spark plugcolor to determine which direction to go for the cure.
Q: What's the difference between a straight-leg booster, adown-leg booster, and an annular-discharge booster? Does booster designaffect jetting?
Demon Carbs responds: The straight-leg booster has, asthe name implies, a straight leg out from the main body with a dischargering above the carburetor's venturi. A down-leg or drop-leg boosterdrops the discharge ring lower into the venturi where it is in thehigher velocity airstream, which will pull more fuel than astraight-leg-style booster. An annular-discharge booster has a largerring with multiple discharge holes rather than the single outlet hole ofa straight-leg or down-leg-style booster. This creates a venturi insidethe main venturi that will create more vacuum signal to pull even morefuel. There are several pros and cons for each type of booster based onthe application they are installed on. If you were to run each style ofbooster in the same carburetor with all else being equal, the down-legbooster would require a smaller jet to flow the same amount of fuel thanwould a straight-leg booster, while the annular-discharge boosterrequires an even smaller jet to flow the same amount of fuel as thedown-leg booster.
Q: How does weather and altitude affect carb jetting?
Demon Carbs responds: Put very simply, the more oxygenthere is in the air, the more fuel--and therefore, the larger the jetsize--the engine needs. As the temperature gets colder or the altitudegets lower, there is more oxygen in the air, so you'll need to go up onthe jet size to add more fuel. Conversely, as it gets warmer or you gohigher in altitude, you'll need to go down on your main jet size.
Q: How do I know what size and type of needle-and-seat touse?
Holley Performance responds: Holley offers a variety ofneedle-and-seat assemblies for its carburetors. The configuration of theneedle-and-seat assembly and its seat size depend on carburetorapplication, cfm rating, and type of fuel bowl used.
Seat size determines the maximum amount of fuel that can flow through itat a given pressure. A bigger seat flows more fuel. Most Holleyperformance carburetors come with Viton-tipped needles. The Viton needledesign is resistant to dirt and conforms nicely to the shape of the seatfor superior sealing. For this reason it is not recommended to use asteel or titanium needle, except for racing. In street and mild raceapplications, a 0.110-inch Viton assembly (PN 6-504) is standard andflows sufficient fuel for carbs up to 850 cfm. Viton PN 6-518-2 has aseat size of 0.120 inch and is standard on carburetors like Holley's 950and 1,000 HP as well as Dominators.
Holley offers steel-tipped inlet needles that are necessary when usingexotic racing fuels or alcohol, or when using benzine or acetoneadditives. A 0.097-inch steel seat size should be used for smallfour-barrel carburetors; a 0.110-inch steel seat size should be used forcarburetors up to 735 cfm; larger seat sizes should be used withcarburetors 750 cfm and larger.
Holley also offers a titanium inlet needle with a 0.150-inch seat. Thisis about as big as you can get. Its design is very responsive to changesin flow rates and has excellent sealing capabilities.