Induction System Basics - Hot Bike Tech

Carbureted Engines
For carbureted engines, replacing the stock air cleaner with a free-flowing unit is an easy way to increase airflow and performance. Various low-restriction aftermarket air cleaners' assemblies are available that easily bolt on to a stock CV carburetor. The job of a performance air cleaner is two-fold: 1) remove contaminants in the air to protect the engine and 2) flow sufficient air for maximum power. Air cleaners vary widely in performance based on design, size, and filter material. Over the years, the K&N-style filter element has proved to be an excellent design that satisfies both requirements. K&N elements use a pleated gauze material wrapped in wire mesh. The pleated design provides a large surface area for high airflow, while the gauze material-when treated with the proper type of oil-does an excellent job of trapping harmful particles that can scratch cylinder walls, piston rings, and valve seats. K&N-style filter elements are washable and rechargable with oil for maintaining optimum flow and filtration.

Increased airflow requires richer carburetor jetting for the additional air. In fact, the stock CV's jetting is too lean, and a performance increase can usually be realized by making jetting modifications while using even a stock air cleaner. As such, always be sure to re-tune your carburetor after installing a high-flow air cleaner. Additionally, regardless of the filter system you buy, make sure that it includes the proper features for connection of any engine breather hoses.

The stock Keihin 40mm CV carburetor that shipped with some Evo and Twin Cam engines is an excellent carb, providing decent performance for mildly modified engines, along with excellent throttle response and fuel economy. However, to realize optimum performance, the carb must be rejetted. Various jet kits are available that include a selection of low and high-speed jets, tapered jet needles, and instructions for many engine combinations.

Although the stock 40mm CV is an excellent carburetor, it has flow limitations. High-revving and large displacement Evo Big Twin and Twin Cam engines can benefit from a higher-flowing carburetor. A carburetor operates by responding to differences between atmospheric pressure and the reduced pressure created by a venturi in the carb's throat. Certain minimum air velocities must be maintained through the carb to achieve optimized air/fuel mixture and fuel atomization for maximum power. Selecting the correct size carb for the application will allow proper intake tract velocities for optimum performance, while an excessively large carb will result in erratic fuel metering, poor fuel atomization, and reduced power and throttle response.

To increase airflow, you have the option of either modifying the stock 40mm CV carb or installing a larger aftermarket carburetor. Trock Cycle and Wood Performance offer various stages of modifications for the CV carb that increase flow and improve throttle response. Harley-Davidson Screamin' Eagle also offers high-flowing 44mm and 51mm CV carbs.

Other high-flowing carbs are available, including the popular S&S and Mikuni. S&S offers butterfly-style Super E, G, and D versions of their carb, while Mikuni has flat slide 42mm, 45mm and 48mm carburetors. Whereas the stock 40mm CV carb maintains airflow velocity and atomization at a constant level using a throttle plate and vacuum slide, S&S carbs use a butterfly (round throttle plate) and Mikunis use a flat rectangular slide that respond directly to throttle position. Optimum carb size is based on engine displacement, rpm, engine combination, and application. But each carb's performance range can overlap the range of a different size carb, so rider priorities can affect the final decision on carb model and size. Nevertheless, some broad guidelines can be established for carb sizing, but remember that engine displacement alone is not the only determining factor for optimum performance.