The Mikuni Corporation manufactures high-performance carburettors and fuel system components for a number of different motorsport vehicles, including water jet skis, motorcycles, and dirt bikes. Because of the different format and design of different engines needed for different vehicles, the specifications for the carburettor, as well as the carburettor design, may change drastically from carb to carb. Knowing the specifications for each type of carburettor, as well as the design of the engine and fuel system you have, is an important part of making sure that the correct carburettor is in use. Also, knowing the performance effect that's desired from the carburettor will often help in determining which carburettor should be used.
The HSR48 carburettor is a strictly-racing designed carburettor, and is not meant for motorcycles that are otherwise built from stock parts. In fact, in order to install the HSR48 carburettor on a Harley Davidson motorcycle, you must purchase a special flange, or the carburettor will sit in a manner which pushes it against the gas tank. The racing specifications on the HSR48 include a main jet size of 190, with a pilot jet size of 25. With a pump nozzle size of 70 and a needle valve size of 4.5, the HSR48 supplies far too rich a supply of fuel and oxygen to an unmodified engine. Providing this enriched fuel supply to a stock engine can cause flooding, which in turn can cause damage to the engine seals if raw fuel sits idly in the engine.
The HSR42 carburettor is another performance-enhancing carburettor made by Mikuni, but this one will work with an otherwise stock engine. With the installation of the HSR42, Mikuni.com reports that you can achieve a ten per cent increase in both power output and engine torque. If you apply the HSR42 carburettor to a motorcycle that has implemented an aftermarket performance exhaust, as well as an upgraded air cleaner, the motorcycle will be able to produce just over a 25 per cent increase in the torque and power output of the engine. This is due to a 42mm carburettor valve that maximises the flow of fuel and oxygen to the engine.
The HS40 carburettor is the smallest carburettor that can be used in motorcycles, and as such is the most conformable to stock engines. With three tunable parts to the fuel jet system including the needle jet, the jet needle, and the main jet, the carburettor is easily able to maintain certain fuel flows at different throttle positions, and most stock motorcycle engines are easily adapted to the carburettor. It's also relatively easy to install because of its size.
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