How to Maintain the Carburetor and Fuel System (5)

Aug 25
08:34

2013

Olivia Tong

Olivia Tong

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Whether you have a Demon or a stock HOLLEYý carburetor, jetting should be fairly close if the carburetor is used in the intended application. For maximum performance, increase or decrease jet size two numbers (primary and secondary) as required. As long as the performance continues to improve, continue increasing or decreasing jet size. At some point, ET or lap times will start to fall off, which means the mixture has been moved past the optimum air/fuel ratio. At that point, move one jet size at a time in the opposite direction until optimum performance is achieved.

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Adjusting the Carb Jetting

Whether you have a Demon or a stock HOLLEYý carburetor,How to Maintain the Carburetor and Fuel System (5)    Articles jetting should be fairly close if the carburetor is used in the intended application. For maximum performance, increase or decrease jet size two numbers (primary and secondary) as required. As long as the performance continues to improve, continue increasing or decreasing jet size. At some point, ET or lap times will start to fall off, which means the mixture has been moved past the optimum air/fuel ratio. At that point, move one jet size at a time in the opposite direction until optimum performance is achieved.

Always jet for performance, not spark plug color. Most high-energy ignitions will leave very little residue on the plugs. With a drag car, plugs can remain bone white so attempting to ýreadý spark plugs is a waste of time. With an oval track car, the plugs will color, but the process takes longer with a high-energy ignition. If the car runs a little too hot, jetting up one or two sizes will alleviate the lean condition without hurting performance, because the ignition will burn off the extra fuel and cool the engine. However, if jet size is increased but the engine appears to be running leaner, a fuel system problem is indicated. If a conventional ignition is used, jetting for best performance is still the way to go, but the plugs will take on a tan color after a short time.

 

Problem-Solving

Generally an engine will pop, miss or surge if it's lean, although an excessively rich condition can cause the same problems. As a rule, cool, dense air requires larger jets, while hot thin air requires smaller jets. Also, whenever a carburetor spacer is added or removed, a camshaft, cylinder head or intake manifold change is made; it will be necessary to retune the carburetor to regain maximum performance.

 

Extensions

Another problem that will cause a stumble or hesitation is when the secondary jets become uncovered, and no amount of accelerator pump tuning will cure it. This occurs when a car leaves so hard that G-force loading on the fuel in the secondary bowl causes it to be pushed up against the rear wall, uncovering the jets. BG Fuel Systems Stainless Steel Jet Extensions will correct this situation. Whenever jet extensions are installed in a carburetor equipped with a secondary power valve, the power valve must be removed and the hole plugged. Jet size must then be increased accordingly.

 

Pump Circuitry

Even when the jetting is within the ball park, this is no guarantee of optimal performance. Performance will be poor if the accelerator pump nozzle (squirter) diameter is incorrect. If a car is sluggish during initial acceleration and a puff of black smoke blows out of the headers when a drag car leaves the starting line, or when an oval track car comes off a corner, the accelerator pump nozzle diameter may be too large. Fuel spilling out of the vent tubes is another possible cause. The latter problem is easily cured by running a rubber hose from one vent tube to another, with a slot cut at the top of the hose. As with jetting, determining the best squirter diameter is accomplished by trial-and-error testing. Simply adjust size up or down for best performance.

 

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