Mikuni has followed up on its successful HS40 Carburetor with the advanced technology second generation performance carburetor called the HSR42. The designation HSR42 denotes a smoothbore racing carburetor with a 42mm venturi for Harley-Davidson applications.
The HSR42 and its big brother the HSR45 were designed with a larger body assembly to fill the need for bigger throttle bores in all-out performance applications. Mikuni designed the new HSR42 carburetor with a smooth pulling 8-roller bearing flat throttle slide assembly. The flat throttle slide design allows an unobstructed venturi at full throttle to flow more air, and it carburetes more precisely at all throttle settings. Most aftermarket carburetors use butterfly throttle valves which obstruct the carburetorís venturi at all throttle openings, creating airflow turbulence and hindering precise air/fuel metering. Mikuniís roller bearing throttle slide offers smoother control and allows the use of a lighter throttle return spring for reduced throttle effort.
Flow bench tests show that the new Mikuni HSR42 and HSR45 are able to flow more air to the engine once installed on the manifold and head than other models of aftermarket big-bore carburetors.
This is because other big venturi carburetors have that restrictive butterfly throttle valve which disrupts and hinders air flow. The HSR42 actually flows more air at full throttle to produce more peak power than other bigger carburetors. While at smaller throttle openings the Mikuniís smaller, yet higher velocity flowing venturi, together with its advance fuel metering system provides a stronger overall power bank with improved ridability.
The HSR42 is also the largest performance carburetor on the market to meet the requirements of the California Air Resources Board for installation on 1340cc Evo model Harley-Davidsons.
Other performance features include a larger capacity float bowl and a higher flowing needle valve assembly to accommodate extended full throttle operation.The Mikuni adjustable accelerator pump provides instant throttle response throughout the RPM range. Mikuni HSR42/45s are available in complete bolt-on kits for most Harley-Davidson models.