Rivera Primo Common Plenum Dual 42mm Mikuni Carburetor Kit (DMK) - Manual (Page 6)

Main system fuel is controlled by the taper from
where it begins to lift out of the needle jet until 3/4
throttle. Raising or lowering the needle will
respectively richen or lean the mixture.
At 3/4 throttle the orifice formed by the needle jet
becomes large enough that the size of the main
jet begins to control fuel flow. Until this point the
main jet has no effect on mixture strength. At full
throttle the needle taper has no influence.
ACCELERATOR PUMP
Mikuni HSR's are fitted with an accelerator pump.
The purpose of the pump is to inject fuel into the
throat of the HSR when the throttle is opened. As
the throttle is opened, especially at low rpm, air
velocity through the carburetor drops and the
mixture naturally leans out. Fuel from the pump
maintains a more correct air-fuel mixture until air
velocity increases.
The accelerator pump can be adjusted to inject
fuel into the throat of the carburetor over a wide
range of throttle openings. The rate at which it
injects fuel can be controlled with different size
pump nozzles. Total fuel volume depends upon
the pump's beginning and end point adjustments.
For the best performance from the accelerator
pump, the nozzle spray should be directed
toward the jet needle.
STARTER SYSTEM
The HSR's starter system is actually a small aux-
iliary carburetor designed to supply a rich mixture
that is required for starting and warm-up. The
starter system works when the choke knob is
pulled out and the throttle is in the closed posi-
tion.
If the throttle is opened with the choke knob
pulled out, air will cease flowing through the
starter system and it will stop delivering a rich fuel
mixture to the engine.
If the engine begins to load up while the starter
system is engaged, it may be cleared by opening
the throttle. When the throttle is closed, the
starter system will resume operation.
GENERAL TUNING PROCEDURES:
MIKUNI HSR CARBURETORS
For the quickest and most accurate results fine
tuning your dual HSR's, we recommend using
"witness" marks, indicating the amount of throt-
tle opening, on the throttle grip and throttle
housing. Use masking tape on the grip and an
indicator mark on the throttle, with the grip tape
marked off in increments of 1/4 throttle open-
ings. In this way, you will be able to identify the
throttle opening where a tuning change may be
required and can adjust the correct carburetor
components.
Your Mikuni HSR is fitted with the tuning parts
we found to work with a majority of engine tun-
ing combinations. However, the tremendous
number of differing exhaust systems and cams
available for Harley engines makes it impossible
to accommodate all possible combinations with
one carburetor set-up. You will probably find that
the HSR will run perfectly on your engine with-
out 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 HSR than in the stock Harley
carburetor. With these parts you will be able to
precisely tailor the HSR42 to your engine's
requirements throughout its rpm and throttle set-
ting 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 HSR, if any are
needed at all, read the section of this manual
describing the various tuning components and
their functional range.
BASIC PRINCIPLES OF
CARBURETOR OPERATION
As air passes through a carburetor, fuel is intro-
duced to obtain the correct air-fuel ratio. This is
accomplished with the help of a venturi. A venturi
is a restriction in the size of the carb body that air
flows through. As air flows through the venturi, its
speed increases because of the smaller area of
the passage. An increase in air speed causes a
corresponding reduction in air pressure. So, at
any given engine speed, the smaller the venturi,
the higher the air speed; the higher the air speed,
the lower the air pressure. HSR carbs use a slide
(technically, a variable venturi) to take maximum
advantage of this relationship.
The primary advantage of slide type carburetor
over a butterfly type is that as fuel is introduced
into the flow of air it is able to mix with all of the
incoming air and not just the portion of air that
passes below the butterfly throttle valve.
Additionally, the HSR incorporates 5 tunable cir-
cuits as compared to the butterfly type carburetor,
which utilizes only 3 circuits. The 5 circuits of the
HSR work progressively as the throttle is opened,
creating seamless throttle response and accel-
eration.
TUNING COMPONENTS
In this segment of the Tuning Section, each of the
HSR's tunable components is outlined and its
function described. Actual tuning procedures are
covered in the next segment: General Tuning
Procedures.
IDLE CIRCUIT (PILOT SYSTEM)
The idle circuit supplies fuel at idle speeds and
has a major influence on fuel flow up to 1/4 throt-
tle. There are two tunable parts in the idle circuit;
the pilot jet and air screw.
1. PILOT JET-controls fuel flow through the
idle circuit.
2. AIR SCREW-controls idle mixture. Turn
the screw out to lean the mixture, turn it in to
richen the mixture. The engine should have a
smooth, steady idle with the screw between 1
turn and 2 turns out from fully bottomed (gently!).
If the engine requires more than 3 turns out, the
pilot jet is too rich (large). If it requires less than
1/2 turn, the jet is too lean (small).
As the throttle is opened, the air screw's position
becomes less important than the sizes of the pilot
jet. A larger pilot jet richens the mixture from just
off idle to 1/4 throttle. A smaller jet leans it. After
changing the pilot jet, it will be necessary to re-
adjust the air screw for best idle.
MAIN SYSTEM
The main system delivers fuel from 1/16 to full
throttle, the idle circuit delivers the majority of
the fuel near 1/16 throttle. The main system
becomes the primary mixture control from about
1/4 throttle.
The main system has three tunable parts:
1. NEEDLE JET-controls mixture from 1/16
to approximately 1/4 throttle (varies with needle
position).
2. JET NEEDLE-controls mixture from 1/8 to
3/4 throttle.
3. MAIN JET-controls mixture from 3/4 to full
throttle.
The jet needle has a constant (straight) diame-
ter section and a tapered section. The outside
diameter of the jet needle and the inside diame-
ter of the needle jet form an orifice through
which all main system fuel must flow.
Until about 1/4 throttle, the constant diameter
section of the jet needle is within the needle jet
and main fuel flow is controlled by the size of
the jet needle.
Notches at the top of the jet needle allow it to be
raised or lowered. Raising or lowering the nee-
dle will determine at what throttle opening the
tapered part of the needle is raised out of the
needle jet.
DUAL DUAL CARB INST CARB INST ALLA ALLA TION TION DUAL DUAL CARB INST CARB INST ALLA ALLA TION TION
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