Rivera Primo Common Plenum Dual 42mm Mikuni Carburetor Kit (DMK) - Manual (Page 7)

The beginning point of the pump stroke is adjust-
ed with the adjusting screw #1 on the white plas-
tic pump lever. To start the pump sooner, back the
screw out. Turn the screw in to make the pump
start its stroke at a larger throttle opening. Dual
carburetors 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 adjust-
ed 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 1/2 throttle.
As a test to see if the accelerator pump is open-
ing at the correct time, screw in the #1 screw
(delaying opening) until there is a noticeable
engine hesitation as the throttle is opened. Then,
back the screw out until the hesitation disap-
pears, this should be the correct opening adjust-
ment.
To set the correct stopping point, screw in the #2
adjuster until the engine hesitates when the throt-
tle is opened. Then, back the screw out until the
hesitation disappears.
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 (richer) for your
particular engine set up.
The initial straight portion of the jet needle affects
the mixture from 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 flat. There may also be some
detonation during part throttle acceleration,
although this can also be caused by other fac-
tors. A lean jet needle will also result in a slow
warm-up.
If part throttle acceleration is flat, install a one
size smaller jet needle and compare the perform-
ance. If acceleration is improved, leave the small-
er jet needle in and take a 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, 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 porcelain around
the electrode should be from a 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 fitted.
RICH CONDITION
While a black sooty spark plug is a sure sign of
richness, there are others that are a bit more sub-
tle. 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,
that 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 mixture strength. Dark gray
with some black is normal for today's lead free
gasolines. If the exhaust color is black, chances
are you can enlarge the size of the jet needle.
A slightly over-rich condition lets a Harley accel-
erate better at very low rpm's and from very low
throttle settings. It may be that you prefer a jet
needle or accelerator pump setting that is slight-
ly on the rich side of the correct range. 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
There is little point in attempting to tune carbure-
tors 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 carburetor tuning difficult. For instance, it is
very difficult to get smooth responsive carbure-
tion with straight and open pipes. If you have any
doubts about the condition of your engine, tune
and test it before beginning what could be a frus-
trating and unproductive effort to fix another prob-
lem with the carburetor.
PILOT SYSTEM
TUNING THE PILOT CIRCUIT
The first step in tuning this carburetor is to get the
idle circuit correctly adjusted. And the first step in
this procedure is to adjust the air screws for the
best idle. Mikuni sets the air screw at 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
re-open it to two turns out from the fully closed
position. The knurled knob above the air cleaner
is the adjuster for the second carburetor. Set
both air screws to the same number of turns dur-
ing tuning.
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 tem-
perature gauge, ride until the oil temperature is at
or near 150 degrees.
With the machine vertical and the engine idling
near 1,000 rpm, adjust the air screws in slowly
until the idle either slows or becomes irregular,
then begin turning them out until the engine again
slows or begins an irregular idle. Count the num-
ber of turns between the too rich and too lean
positions.
Set the air screw mid-way between the too rich
and too lean positions. You may further refine the
air screw position with further riding experience,
but this will provide you with the perfect idle mix-
ture setting.
If you allow the engine to get too hot during the
air 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 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.
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 step.
ACCELERATOR PUMP SYSTEM
TUNING THE ACCELERATOR PUMP
The purpose of the accelerator pump is to inject
a metered amount of fuel into the engine at small
throttle openings. This is the time where air veloc-
ity is the lowest and this additional amount of fuel
enables the engine to rev up smoothly and quick-
ly. Mikuni HSR's use an accelerator pump in
order to achieve maximum fuel economy during
cruise throttle settings and still create a rich
enough condition for brisk throttle response.
Your dual HSR carburetors have the accelerator
pump installed in only one carburetor, this is the
carburetor closest to the engine. The accelerator
pump is factory set and should not require adjust-
ment under normal conditions.
The HSR's accelerator pump can be adjusted to
begin and end operation in response to the fuel
requirements of your particular engine.
DUAL DUAL CARB INST CARB INST ALLA ALLA TION TION DUAL DUAL CARB INST CARB INST ALLA ALLA TION TION
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CAUTION
NOTE