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Air Filters, Carburetors & Jetting


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The late model air filters on these bikes do a decent job of filtering in-coming air and silencing intake noise, but they restrict air flow. A free-flowing air filter will allow a V-Twin to pass more air for improved combustion. Flow bench tests have shown that the H-D Screaming Eagle air filter is capable of flowing all the air that a stock CV carburetor can flow. S & S also has an efficient air filter and there are many others available to suit individual taste. Due to the passage of more air, the installation of a free-flowing air filter may cause the air/fuel ratio to become "lean" (too much air for the amount of gas that the carburetor meters into the engine) which means that jetting adjustments should be made to the carburetor.

Changing the carburetor jetting is not difficult but it does require some tuning experience for accurate dial-in. Proper jetting can transform an average, sluggish engine into a highly responsive powerplant that if a real delight to ride. The sad truth, however, is that the majority of bikes on the road today are not properly jetted, especially factory-stock bikes. In the case of late models, this is not the fault of Harley-Davidson, but rather the E.P.A. who mandates very strict emission controls.

For maximum power, the proper air/fuel ratio for normal engine operation is between 13:1 and 14:1. This is slightly rich and is not the "stoichiometric" ratio 14.7:1 (ideal chemical mix). The "ideal chemical mix" does not produce max power in most engines. The ratio simply means that every 1 part of gas requires 13 or 14 parts of air for optimal combustion. More than 14 parts of air results in lean mixtures and less than 14 parts of air results in rich mixtures. Newer engines that spit, cough, hesitate and backfire through the carburetor are suffering from "EPA" jet settings.

Most late-model Evo Harleys and the newer twin cams are factory-equipped with CV (Constant Velocity) carburetors that are adequate for normal street operation and for mildly hopped-up engines, but major horsepower increases require changing to a higher capacity carburetor. For example, if you add a "hotter" cam, have the heads "ported" for increased flow and add an efficient exhaust system, the factory carburetor may bottleneck air flow and limit the power gains that the modifications could produce. All components must work in harmony for optimal power output. Conversely, if you just add a larger carburetor to a stock engine, there will be little, if any, power gain because stock heads don't flow well enough to measurably benefit from the carburetor's higher flow capacity (certain Buell models are exceptions).

The carburetor must be "matched" with the engine's air flow abilities. Engine air flow velocity is equally important with air flow volume. The flow velocity must be as high as possible for responsive torque at low-to-mid rpm while the flow volume must be sufficient to supply the engine with all of the air it needs to attain maximum high-rpm power (these two flow factors are at odds with one another and cause design compromises). In other words, the size of the carburetor venturi, or opening, must be large enough to flow the required volume without causing low flow velocity. This also applies to the intake manifold, both head ports, valves, combustion chamber and the exhaust system. They all must function in harmony for optimal power throughout the powerband (the engine's operational rpm range). Regardless of the flow volume, if flow velocity drops below a critical level, throttle response and torque will suffer.

Any biker of modest experience who is contemplating a carburetor change is urged to carefully research the available options before taking a plunge that might result in disappointing performance. For power purposes, a carburetor should not be selected on the basis of cool appearance, slick advertising or as a mimic of another bike. Many bike professionals will share their experience with others just for the asking.