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HSR42/45/48 Carburetors

(General Tuning Procedures)

General Tuning Procedures

Your Mikuni HSR42 is itted with the tuning parts we found to

work with a majority of engine tuning combinations. However, the

tremendous number of differing exhaust systems and cams available

for Harley engines make it impossible to accommodate all possible

combinations with one carburetor set-up. You will probably ind that

the HSR42 will run perfectly on your engine without exchanging any

parts. But if it doesn’t you may alter its tuning to suit your engines

needs by following this guide.

There are many more replaceable parts that affect tuning in the

HSR42 than in the stock Harley carburetor. With these parts you will

be able to precisely tailor the HSR42 to your engines requirements

throughout its rpm and throttle setting range.

Each tuning system is easy to modify and diagnose, but only when

you understand what each system does and how it works. Before

making any alterations to the HSR42, if any are needed at all, read

the section of this manual describing the various tuning components

and their functional range.

There is simply no point in attempting to tune any carburetor

unless the engine is completely sound. Valves and rings must seal

properly, the ignition timing must be correct and the spark plugs

clean and gapped. Some exhaust systems may also make carbu-

retor tuning dificult. For instance, it is very dificult to get smooth

responsive carburetion with straight and open pipes. If you have nay

doubts about the condition of your engine, tune and test it before

beginning what could be a frustration and unproductive effort to ix

another problem with the carburetor.

Pilot System - Tuning the Pilot Circuit

The irst step in tuning this carburetor is to get the idle circuit

correctly adjusted. And the irst step in this procedure is to adjust

the air screw ir the best idle. Mikuni sets the air screw two turns out

during assembly. This is the position we have found to be right most

of the time. If the screw position had been altered, gently bottom it

and reopen it to two turns out from the fully closed position.

Next, ride the bike until the engine is at it normal operating

temperature. This may require several miles a highway speeds. If

you have an oil temperature gauge, ride until the oil temperature is

at or near150.

With the machine vertical and the engine idling near 1,000 rpm,

adjust the air screw out until the engine again slows or becomes

irregular, then begin turning the screw out until the engine again

slows or begins an irregular idle. Count the number of turn between

the too rich and too lean positions.

Set the air screw-mid-way between the too rich and too lean posi-

tions. You may further reine the air screw position with further riding

experience, but this will be vary close to the perfect idle mixture


If you allow the engine to get too hot during the air screw

adjustment procedure, the resulting adjustment will probable be on

the lean side of correct. If you have a large fan, use it while adjusting

the mixture. If you do not have one, you may need to take him out

for a short ride to cool the engine back to normal temperature.

NOTE: IF the best idle is achieved with the air screw less than 1/4

turn out, the pilot jet is too small and will need to be changed.

On the other hand, if the air screw must be more than three turns

out for best idle, the pilot jet is too large and will need to be changed.

Once you have a good idle with the air screw within its range, you

may proceed to the next stage: Tuning the Jet Needle.

Main System - Tuning the Jet Needle

(Initial straight portion)

It is unlikely that you will need to change the jet needle from the

one supplied in your HSR42. However, in case you do, you should be

aware of how it works and how to tell if the one you have is too large

(leaner) or too small for your particular engine set up.

The initial straight portion of the jet needle affect the mixture

from the idle to approximately 1/4 throttle, at which point the needle’s

tapered portion takes over.

Lean Condition

If the jet needle is too lean (large), part throttle acceleration will be lat.

There may also be some detonation during part throttle

acceleration, although this can be caused by other factors. A lean

jet needle will also result in a slow warm-up.

If part throttle acceleration is lat, install one size smaller jet

needle and compare the performance. If acceleration is improved, leave

the smaller jet needle in and take a fairly long ride at steady speeds to

give the spark plugs time to color evenly.

Take a spark plug wrench with you and after a few miles at steady

speeds, stop and remove a plug for inspection. Be careful as you stop

not to operate the throttle. The extra fuel from the accelerator pump can

cause a false plug reading. The body of a spark plug can be from light

gray to brown to dark gray. If the plug body is black and has a sooty

appearance the jet needle is probably too rich and a larger one will need

to be itted.

Rich Condition

While a black sooty spark plug is a sure sign of richness, there are

others that are a bit more subtle. If your engine responds crisply at low

throttle when it is cold, chances are the jet needle is one size smaller than

it needs to be. Assuming, of course, that the idle circuit is correctly tuned.

Poor fuel mileage is another sign of richness and because of the way

most of us ride our Harleys, the richness is usually the result of a jet

needle that is too small. The color of the end of the exhausts is a sign of

free gasoline. If the exhaust color is black, chances are you can enlarge

the size of the jet needle.

It may be that you prefer a jet needle that is slightly on the rich side

of the correct range. A slightly over-rich condition lets a Harley accelerate

better at very low rpm’s and from very low throttle settings. Be aware that

you will lose some fuel economy if you choose to do this.

Tuning the Jet Needle

(Tapered portion)

Like the idle circuit of the jet needle, the tapered portion has its

range of operation. the tapered portion has an increasing effect on fuel

mixture from about 1/4 throttle opening as the tapered section leaves

the opening of the needle jet/ between 1/2 and 3/4 throttle the jet

needles inluence is the greatest and it controls most of the fuel low.

All jet needle and main jet testing should be done with the engine

near the middle of its rpm range. Start acceleration tests at about 50

mph. The best jet needle position will give the strongest acceleration.

With the engine at operating temperature, accelerate at 1/2 to 3/4

throttle, in top gear from 50mph or so. If acceleration seems soft or lat

and the engine is slow to respond when the throttle is quickly opened

from 1/2 to 3/4 throttle position, the mixture is too lean. Raise the jet

needle one notch and repeat the test.