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HSR42/45/48 Carburetors

(General Tuning Procedures)

On the other hand, if acceleration is crisp but the engines hesitated or

staggers as the throttle suddenly shout down from 3/4 to 1/2 throttle, the

mixture is too rich. Lower the jet needle one notch and repeat the test.

The jet needle postilion will be near correct when acceleration is crisp

at mid rpm yet the engine does not load up during throttle shut down.

Tuning the Main jet

You could in fact remove the main jet from our Mikuni HSR42 and the

engine would run ine until the throttle was near the 3/4 mark. The jet

needle and needle jet restrict the amount of fuel getting into the engine

until about that point. There is no point in changing main jets if a mixture

problem exists below the 3/4 throttle position.

The main jet is the last jet you need to deal with and is the

easiest to get right. IF you have the room, the most effective method for

getting the main jet right is to measure the time required to accelerate

between two points. The start and end markers should be spaced so that

starting at about 35 mph at the irst marker will have you going past the

second at near 55 mph.

Set up markers that are far enough apart, on a safe road, to meet

the conditions mentioned above. When pass the irst marker roll the

throttle fully open and have a friend (it’s easier with help) measure the

time need to her to the second marker. The jet that gives the shortest

time is the correct jet. This method is simple but effective.

Tuning the Accelerator Pump

The beginning point of the pump stroke is adjusted with the adjusting

screw #1 on the white plastic pump lever. To start the pump sooner, back

the crew out. Turn the screw in to make the pump start its stroke at a

larger throttle opening. Most engines perform best if the pump begins its

stroke as soon as the throttle is moved from the idle position.

The end of the accelerator pump stroke is adjusted by the adjusting

screw #2 located on the top of the carburetor just behind the pump lever.

Best performance is generally achieved when the pump stroke ends

before 3/4 throttle.

For best results, the accelerator pump nozzle should be pointed directly

at the jet needle. The nozzle is held in place by the friction of an o-ring

and can be turned easily with a pair of long nosed pliers, Nozzle adjust


ment should be made with a minimum of pump strokes to avoid looding

the engine.

Servicing - HSR42/42 Carburetors

There are very few moving parts in the HSR series carburetor.

This carburetor will not require much servicing, but there are a

few items to be mentioned to assure good performance season after



If the motorcycle is to be stored for any length of time the

carburetor loat bowl should be drained.


The loat drain plug should be removed periodically and cleaned

of any sediment that may have settled during long periods of use.


DO NOT drill or modify any part of this carburetor for any reason

and the result will surely be more problems. Refer to this Tuning

guide to correct any problems that you may have.


If a jet or passage does become plugged, use only carburetor

cleaner and compressed air. DO NOT push a drill or any other

object through the jet or passage to clean them.


If the air ilter is dirty and requires servicing, follow the steps found

on the next page. Typical service is done after about 2,000 miles

of use.


The fuel level in the carburetor will probably never need adjusting;

however, if a problem has been diagnosed as a fuel level that is

too high or too low, adjustment can be done as shown.

Nozzle adjustment should be made with a minimum of pump strokes

to avoid looding the engine.