MIKUNI HSR42 SMOOTHBORE CARBURETTOR

Mikuni’s Research and Development department has spent many months improving its HS series carburettors to meet all rider requirements. The first step was a larger body to accommodate larger choke holes in high-output performance applications; the second step was smoother and easier throttle operation. This resulted in the design of the HSR42 carburettor. A key feature of the HSR42 is the flat slide valve assembly with roller bearings. Unlike butterfly valves, which obstruct the carburettor venturi tube at all throttle openings and create airflow turbulence, making precise air/fuel metering difficult, the flat slide valve allows an unobstructed venturi tube at full throttle, enabling greater airflow and more precise carburation across all throttle positions. The roller bearings on the slide valve provide a light throttle action, offering smoother control and allowing the use of a very light return spring. With its 42 mm venturi, the HSR42 flows 20% more air than the HS40, making the HSR42 an ideal carburettor 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 carburettor for original or lightly modified Sportster Evolution models). Flow bench tests by Jerry Branch show that the Mikuni HSR42, while retaining a smaller venturi tube size than some other popular carburettors, is capable of flowing more air to the engine once installed on the manifold and head. The Mikuni HSR42 actually flows more air at full throttle to produce higher peak power. At partial throttle opening, the smaller Mikuni, with higher velocity airflow through the venturi tube, combined with its advanced fuel metering system, provides a broader powerband with better throttle response. Other performance features of the HSR42 include a larger capacity float bowl and a taller needle valve assembly to accommodate extended full-travel throttle use. Additionally, the float valve design incorporates a lever that prevents the float bowl from overfilling or the needle valve from flooding. This eliminates the need for drain and vent tubes in the carburettor body, giving the carburettor a cleaner appearance. An adjustable accelerator pump provides instant throttle response across the entire rev range. The kits fit throttle bodies and idle cables on Big Twin models from 1990–2017. Installation instructions and hardware are included with each kit. Mikuni has developed several kits for special applications, as well as “Easy Kits” that utilise original manifolds, choke cables, air filters, and throttle cables.
 
 Throttle and idle cables for use with Mikuni HSR42/HSR45 carburettors. The HSR42 and HSR45 carburettors have the cable wheel on the left-hand side of the carburettor and use the cables as in the table above. The HSR42 and HSR45 carburettors have the cable wheel on the right-hand side of the carburettor and are used for models from 1990 to the present with original cables as used on Big Twin models. Most cables are available in black vinyl or braided steel.


Specifications


Pack
1


HS Code
8714109090


Origin
JP


Motorcycle family
Dyna | Softail | Touring