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Top 20 Carb Questions Answered

We polled the experts at Holley, Demon and Edelbrock fro responses to our most probing induction queries.

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Factory production vehicles haven't rolled off the Big Three's assemblyline with new carburetors since the '80s, but it's still the mostpopular choice of induction among car crafters. Don't get us wrong; weunderstand that the latest and greatest computerized fuel-injectionsystems are getting more affordable and enable today's performancetuners to tame mega-horsepower engines for docile street use, but nomatter how you look at it, carburetors will always be cheap andeffective if you know how to properly tune them. But carb experts--theguys who spend all day on the phones with real-world users--tell us itnever ceases to amaze them how many people are completely lost when itcomes to understanding the nitty-gritty details of how a carb reallyworks and how to tune one for maximum performance.

Over the years we've shown our readers everything from how to rebuildcarburetors to how to live with one every day. This time we're going astep further by directly probing minds from Holley Performance, DemonCarburetion, and Edelbrock for answers to 20 of the most commoncarburetion questions from basic to advanced. Even you expert carb gurusout there might learn something new.

Q: Does a bigger carb make more power? What's thelimit?

Demon Carbs responds: Larger carburetors can make morepower on a dynamometer, but this can also result in slower accelerationand lower efficiency of fuel atomization. Generally we find that you'rebetter off leaning towards a slightly smaller carburetor than a largerone, as it'll operate more efficiently and provide better accelerationfor the vehicle.

Q: Where should I hook up my vacuum advance?

Edelbrock responds: If you're unsure of which port touse, connect the vacuum line to the timed vacuum port. This has novacuum at idle and won't affect idle quality. However, if yourapplication requires vacuum at idle, you'll want to connect the line toa manifold vacuum port. The most detrimental effects of using the wrongport is an increase or decrease of rpm and poor idle characteristics.Generally the distributor vacuum line goes to the timed port.

Q: What's the best way to tune the accelerator-pumpcircuit?

Holley Performance responds: The accelerator-pumpsystem consists of three main components: the pump diaphragm, the pumpcam, and the pump nozzle. This is the carburetor system that is mostresponsible for having good, crisp, off-idle throttle response. Itspurpose is to inject a certain amount of fuel down the throttle boreswhen the throttle is opened to smooth the transition between the idleand main circuits so that no stumble, hesitation, or sluggishness willbe evident during this transition phase.

The first adjustment to check is the clearance between the pumpoperating lever and the pump diaphragm cover's arm, at wide-openthrottle. This clearance should be around 0.015 inch. This ensures thatthe pump diaphragm is never stretched to its maximum limit at wide-openthrottle, which will cause premature pump failure. Next, make sure theaccelerator-pump arm is activated the moment that the throttle begins tomove, which ensures instantaneous pump response. These adjustments canbe made by simply turning the accelerator-pump adjusting screw locatedon the accelerator-pump arm together with the pump override spring andlocknut.

The amount of fuel delivered by one accelerator-pump stroke isdetermined by the pump's capacity and the profile of the pump cam. Thetime it takes for that fuel to be delivered is controlled by thepump-nozzle size. A larger pump nozzle delivers the fuel much soonerthan a smaller pump nozzle. During acceleration tests, if you noticethat the car first hesitates and then picks up, it's a sure bet thepump-nozzle size should be increased. A backfire (lean condition) onacceleration also calls for a larger pump-nozzle size. Conversely, ifoff-idle acceleration does not feel crisp or clean, the pump-nozzle sizemay need to be decreased. Holley accelerator-pump nozzles are stampedwith a number that indicates the drilled pump hole size. For example, apump nozzle stamped "35" has a 0.035-inch orifice. Pump nozzle sizes areavailable from 0.025 to 0.052 inch. Whenever a 0.040-inch or largeraccelerator-pump nozzle is installed, the "hollow" pump-nozzle screw (PN26-12) should also be used. This screw allows extra fuel to flow to thepump nozzle, assuring that the pump nozzle itself will be the limitingrestriction in the accelerator-pump fuel-supply system.

When changing the pump nozzle, it's best to jump three sizes. Forexample, if there's an offline hesitation with a No. 28 pump nozzle, trya No. 31 pump nozzle. If you must use a No. 37 (0.037-inch) or largerpump nozzle, then also use a 50cc pump. A 50cc accelerator-pumpconversion kit is available under Holley PN 20-11 when maximum pumpcapacity is desired.

Once nozzle size has been selected, the accelerator-pump system can befurther tailored with the pump cam. Holley offers an assortment ofdifferent pump cams, each with uniquely different lift and durationprofiles that are available under Holley PN 20-12. The cam profileaffects the movement of the accelerator-pump lever and, subsequently,the amount of fuel delivered by the pump nozzle.

Installing a pump cam is a simple matter of loosening one screw, placingthe new pump cam next to the throttle lever, and tightening it up. Thereare two or three numbered holes in each pump cam. Placing the screw inposition number 1 activates the accelerator pump a little early,allowing full use of the pump's capacity. Generally, vehicles thatnormally run at lower idle speeds (600 or 700 rpm) find this positionmore useful because they can have a good pump shot available comingright off this relatively low idle. Positions number 2 or 3 delay thepump action, relatively speaking. These two cam positions are good forengines that idle at 1,000 rpm or above. Repositioning the camcompensates for the extra throttle rotation required to maintain therelatively higher idle setting. Pump-arm adjustment and clearance shouldbe checked and verified each and every time the pump cam or pump-camposition is changed.

Q: How do I choose the right size carburetor for myengine?

Edelbrock responds: A simple formula can assist youwith this. Multiply your cubic-inch displacement by the maximum rpmlimit and then divide by 3,456 to represent the volumetric efficiency.The result is the amount of cfm the engine requires at the maximum rpmlimit.

Example:
350 ci x 6,000 rpm = 2,100,000
2,100,000 / 3,456 = 608cfm
A 600-cfm carb would would be sufficient.

Q: Is air pushed or sucked into a carburetor?

Demon Carbs responds: Unless you are working with aforced-induction system (centrifugal supercharger or a turbochargersystem) that forces air through the carburetor, air is pulled throughthe carburetor based on the engine's vacuum signal.

Q: Where should the fuel-pressure regulator be located and whattype should I use?

Demon Carbs responds: The pressure regulator should bemounted as close as possible to whatever you're feeding (e.g.,carburetor, nitrous system). The farther away the regulator is, thelonger it will take to open and close in response to demand, which cancause spikes in fuel pressure. The type of fuel pump you're using alongwith the fuel requirements of the engine will be the determining factorsfor the type of regulator you'll use.

Q: How does a metering rod work and how do I tune withone?

Edelbrock responds: Fuel must pass through therestriction created by the main jet and metering rod before entering theengine. The rod extends through the jet, reducing the amount of area forfuel flow. Similar to power valves, metering rods add fuel when acertain engine load or vacuum drop is reached. The two metering rods inan Edelbrock carb have stepped ends that protrude into the main jets andrestrict or increase the flow of fuel through the orifice proportionalto engine load. If the diameter of the rod is large, then fuel flowthrough the jets is more restricted than if the rod were small.Specially tuned springs are used to adjust the vacuum levels required toactuate the rods under various load conditions. There is no"pre-determined" combination of rods and jets for any given engine. Eachengine may require a different fuel curve. The best thing to do is runthe carburetor on the engine right out of the box, then check the sparkplugs and see how they look. You need to have a golden brown color onthe plugs. If they are too white or too black, you need to makeadjustments.

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