(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.
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.
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
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.