Mikuni's Research and Development Department has spent many months to improve the HS carburetor series to meet all the street riders performance needs. First step was a larger body to fill the need for bigger throttle bores in all-out performance applications; step two had to be a smoother and easier throttle operation. This resulted in the design of the HSR42 carburetor. A major feature of the HSR42 is the flat throttle slide assembly with roller bearings. Unlike butterfly throttle valves, which obstruct the carburetor's venturi at all throttle openings, creating airflow turbulence and hindering precise air/fuel metering, the flat throttle slide allows an unobstructed venturi at full throttle, thus flowing more air and carbureting more precisely at all throttle settings. The roller bearings on throttle slide provide a light throttle action, which offers smoother control and allows the use of a very light throttle-return spring. With its 42mm venturi, the HSR42 flows 20% more air than the HS40, making the HSR42 an ideal carburetor for Evolution Big Twin and Twin Cam engines employing other performance modifications such as big bore kits, cams, pistons and an exhaust system (Mikuni still recommends the smaller HS40 carburetor for stock and mild tuned Evolution Sportsters). Flow bench tests by Jerry Branch show that the HSR42 Mikuni, while still being smaller in venturi size to some other popular carburetors, is able to flow more air to the engine once installed on the manifold and head. The HSR42 Mikuni actually flows more air at full throttle to produce more peak power. At partial throttle openings, the Mikuni's smaller, yet higher velocity flowing venturi, together with its advanced fuel-metering system provides a stronger overall powerband with improved ridability. Other performance features of the HSR42 include a larger capacity float bowl and a higher flowing needle-valve assembly to accommodate extended full throttle operation. Also, more positive action of the high leverage float valve prevents the possibility of the float bowl over filling or the needle-valve flooding. This eliminates the need for overflow and vent tubes on the carburetor body, giving the carburetor a cleaner look. An adjustable accelerator pump provides instant throttle response throughout the RPM range. Kits work with 1990 thru 2017 style Big Twin throttle and idle cables. Other features are instant engine response, improved rider throttle control, more mid range power for faster acceleration and passing with more confidence, good gas mileage and years of dependable service. Installation instructions and hardware are included with each kit. Mikuni developed several kits for special applications as well as "Easy Kits" that use stock manifolds, choke cable, air cleaner and throttle cable.
Throttle and idle cables for use with Mikuni HSR42, and HSR45 carburetors Early style HSR42 and HSR45 carburetors have the cable wheel on the left side of the carburetor and use cables as in the table above. Late style HSR42 and HSR45 carburetors have the cable wheel on the right side of the carburetor and use 1990 or later OEM style cables as used on Big-Twin models. Most cables are available in black vinyl or braided steel.