Rivera Primo Mikuni 42mm Carb - SH & XL - Manual (Page 6)

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If you allow the engine to get too hot during the pilot screw
adjustment procedure, the resulting adjustment will probably
be on the lean side of correct. If you have a large fan use it
while adjusting the idle mixture. If you do not have one, you
may need to take time out for a short ride to cool the engine
back to normal temperature.
Remember, if best idle is achieved with the pilot screw less
than 1/4 turn out, the pilot jet is too large or the pilot air jet is
too small. One or the other will need to be changed.
On the other hand, if the pilot screw must be more than three
and a half turns out for best idle, the pilot jet is too small (or the
pilot air jet is too large). Either will need to be changed.
If the pilot screw's best idle position is outside the 1/4 to 3-1/2
turn range, the carburetor's mixture will be either too lean or
too rich as the throttle is just being raised off the idle position.
When you have a good idle with the screw within this range,
you may proceed to the next stage: tuning the needle jet.
Pilot air jet is selected by riding at 15 to 30 mph. If the engine
surges or detonates (lean), the pilot air jet should be reduced
in size. Example: 1.1 to 1.0. If on the other hand the engine
misfires or there is after burn from the exhaust (rich), it would
indicate the pilot air jet is too small. If the air jet is changed, the
pilot screws should be re-checked and adjusted if needed.
As mentioned earlier, the idle circuit has an important effect on
mixtures up to quarter throttle. However, the idle system's
effect on mixture strength overlaps the effect of the needle jet
in this range. If the idle circuit is incorrectly adjusted, it will not
be possible to get the needle jet correct.
TUNING THE NEEDLE JET
It is unlikely that you will need to change the needle jet from the
one supplied in your Mikuni 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 (richer) or too small for your particular
engine set up.
The needle jet's effect on mixture is limited from about 1/16
throttle, where the main system begins to deliver fuel, to 1/4
throttle, when the tapered section of the needle begins to
emerge from the mouth of the needle jet.
LEAN CONDITION
If the needle jet is too (lean) small, part throttle acceleration will
be flat. There may also be some detonation during part throttle
acceleration although this can be caused by other factors. A
lean needle jet will also result in a slow warm up.
If part throttle acceleration is flat, install a one-size larger nee-
dle jet and compare the performance. If acceleration is
improved, leave the larger jet 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 & after a few miles at steady
speeds, stop & remove a plug for inspection. Be careful as you
stop not to operate the throttle. The extra fuel from the accel-
erator pump can cause a false plug reading. The body of a
spark plug can be from light grey to brown to dark grey. If the
plug body is black & has a sooty appearance then the needle
jet is probably too rich & a smaller jet is needed.
RICH CONDITION
adjustment during tuning or other handling. If you have doubts,
check the float level and re-adjust if needed. A float level that
is too high makes fine tuning of the idle circuit impossible. Too
low will have a similar effect on mid-range tuning.
GENERAL TUNING PROCEDURES:
MIKUNI HSR42 CARBURETOR
Your Mikuni HSR42 is fitted with tuning parts we found to work
with a majority of engine tuning combinations. However, the
tremendous number of differing exhaust systems & cams avail-
able for Harley engines make it impossible to accommodate all
possible combinations with one carburetor set-up. You will
probably find that the HSR42 will run perfectly on your engine
without exchanging any parts. If it doesn't, alter tuning to suit
your engine's needs by following this guide.
There are many replaceable parts that affect tuning in the
HSR42. With these parts you will be able to precisely tailor the
HSR42 to your engine's requirements throughout its rpm &
throttle setting range. Each tuning system is easy to modify &
diagnose, but only when you understand what each system
does & how it works. Before making any alterations to the
HSR42, if any are needed at all, read the section of this man-
ual describing the various tuning components & their function-
al range. There is 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 & the spark
plugs clean & gapped. Some exhaust systems also make car-
buretor tuning difficult. It is almost impossible to get smooth
responsive carburetion with straight or open pipes. If you have
doubts about the condition of your engine, tune & test it before
beginning what could be a frustrating & unproductive effort to
fix another problem with the carburetor.
TUNING THE IDLE CIRCUIT (PILOT SYSTEM)
The first step in tuning any carburetor is to get the idle circuit
correctly adjusted. The first step in this procedure is to adjust
the pilot screw position for best idle. Mikuni sets the pilot screw
at three turns open during assembly. This is the position found
to be right most of the time. If the screw position has been
altered, gently bottom it & re-open three turns out from the fully
closed position.
Next, ride the bike until the engine is at its normal operating
temperature. This may require several miles at highway
speeds. If you have an oil temperature gauge, ride until the oil
temperature is at or near 150 degrees.
With the machine vertical & the engine idling near 1,000 rpm,
adjust the pilot screw in one half turn at a time until the idle
either slows or becomes irregular. The pilot screw is now in too
far & the idle mixture is too lean. Pause for a few seconds at
each half turn to allow the engine to settle down and give a
clear indication of the mixture strength. Now, begin turning the
screws out in half turn intervals until the engine again slows or
begins an irregular idle. Count the turns between the too lean
and too rich positions. Set the pilot screw mid-way between the
too-lean and too-rich positions. You may further refine the pilot
screw position with further riding experience but this will be
very close to the perfect idle mixture setting.
SHOVELHEAD/IRON HEAD XL
Installation Instructions