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Tech Answer

How do I determine the proper size jets for my carb?

Whether you have a stock, Holley, or a flow-modified Barry Grant carburetor, jetting should be fairly close if the carburetor is used in the application for which it was intended. For maximum performance, increase or decrease jet size two numbers (primary and secondary) as required. So long as performance improves, 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. 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 "read" 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. 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. 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, hot thin air requires smaller jets. Also, whenever a carburetor spacer is added or deleted, or a camshaft, cylinder head, or intake manifold change is made, it will be necessary to retune the carburetor for maximum performance.

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