Cars & Vehicles Auto Racing

Holley Carb Tips

    • Dual Holley four-barrel carburetors sit atop a high-rise intake.car engine image by itsallgood from Fotolia.com

      Holley carburetors mean high performance, and, even with today's emphasis on electronic fuel injection, they still are favorite choices for performance applications. The Holley carburetor is simply constructed but still achieves a high level of "tunability," and, according to Car Craft, it is an excellent choice for the performance-minded enthusiast aiming to improve a vehicle's track times while staying within a budget. Most Holley carburetors work well out of the box, but basic tuning tricks can optimize their performance.

    Secondary Diaphragm

    • Holley carburetors that use vacuum-actuated secondaries require opening the vacuum diaphragm assembly to change the spring that controls the rate of opening. This rubber diaphragm is notorious for tearing upon reassembly. To avoid this problem and make changing the spring simpler, a quick-change diaphragm kit is available and eliminates the need to remove the diaphragm to change the spring.

    Gaskets

    • Holley carburetors often use plain cork or paper gaskets that either tear when the carburetor is dismantled or shrink during long periods of disuse. Holley offers blue gaskets of much more durable material that eliminate this problem and can be installed easily in minutes.

    Pump Cam

    • Holley carburetors use a nylon cam that actuates the accelerator pump and controls the amount of extra fuel added upon initial acceleration. There are several different-size pump cams that can be used to fine-tune the amount of fuel provided by the pump, helping to correct or improve throttle response.

    Sizing

    • Choosing the correct Holley carburetor for your vehicle involves more than just picking a big model for your engine. To get the approximate size needed for your engine, multiply the engine's displacement by the maximum rpm it is intended to reach. You then divide this number by 3,456 to get the approximate number size and choose the carburetor size that is closest to that number without going under the number. It is better to go a little over the computed number than over so the engine does not become starved for fuel and air at high rpms, thus lowering peak power.

    Secondaries

    • Secondaries increase the flow capability of the carburetor and are a crucial part of its ability to produce power. Improperly matched secondary operation can cause poor performance and reduced fuel efficiency. Vacuum-operated secondaries are preferable to mechanically operated secondaries for street use, and they perform best on larger vehicles with automatic transmissions. Mechanical secondaries are oriented more toward higher-performance applications, and they should be used with manual transmissions, engines using large cams or vehicles intended mainly for drag-strip use.

    Jets

    • Jets control how much fuel flows into the carburetor bowls, and how fast it flows. Proper size is found through trial and error. Too small of a jet size will cause stumbling at acceleration, with no smoke from the exhaust. Too large of a jet size will cause stumbling, with black smoke coming from the exhaust. The correct jets will produce sharp acceleration without any smoke.



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