Rivera Primo Mikuni 42mm Carb - SH & XL - Manual (Page 5)

5
MIKUNI HSR42 CARBURETOR
HSR42'S TUNING CIRCUITS & HOW THEY
WORK:
IDLE CIRCUIT (PILOT SYSTEM)
The idle circuit supplies fuel at idle speeds & has a big influ-
ence on fuel flow up to 1/4 throttle. There are three tunable
parts in the idle circuit:
1). PILOT JET-controls maximum fuel flow thru the idle circuit.
2). PILOT AIR JET-controls the maximum amount of fuel that
will flow thru the pilot jet by allowing a higher (smaller air jet) or
lower (larger air jet) vacuum signal at the pilot jet.
3). PILOT SCREW-controls how much fuel is allowed to enter
the carburetor venturi.
The pilot screw is used to control idle mixture. Turn the screw
out to richen the idle mixture. Turn it in to lean the mixture. The
engine should have a smooth, steady idle with the screw
between 1/4 and + 3-1/2 turns out from fully bottomed (gently!).
If the engine requires more than three turns out, the pilot or
pilot air jet may be too lean. If it requires less than 1/4 turn, it
may be too rich.
As the throttle is opened the pilot screw's position becomes
less important than the size of the pilot & pilot air jets. A larger
pilot jet richens the mixture from just off-idle to 1/4 throttle. A
smaller one leans it. A change in pilot air jet has the reverse
effect. A larger pilot air jet leans the mixture and a smaller one
richens it. The pilot jet & pilot air jet have slightly different
effects on mixture strengths. These effects are discussed in the
"General Tuning Procedure”.
The idle circuit is adjusted by changing either the pilot or the
pilot air jets. A one-size larger pilot jet will have nearly the same
effect as a one-size smaller pilot air jet. It is generally easier to
change the air jet since it is more accessible.
After changing either jet, it will be necessary to re-adjust the
pilot 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 important 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 1/4 to 3/4 throttle.
3). MAIN JET-controls mixture from 3/4 to full throttle.
The jet needle has a constant diameter section & a tapered
section. The diameter of the needle & the inside diameter of
the needle jet form an orifice thru which all main system fuel
flows.
Until about 1/4 throttle, the constant diameter section of the
needle is within the needle jet & main system fuel flow is con-
trolled by the size of the needle jet. Needle jets are available
with different inside diameters. A larger needle jet richens mix-
ture within its range of operation.
Notches at the top of the needle allow it to be raised or low-
ered. Raising or lowering the needle will determine at what
throttle setting the tapered part of the needle is raised out of
the needle jet. Main system fuel flow is controlled by the nee-
dle's taper from where it begins to lift out of the needle jet until
about 3/4 throttle. Raising or lowering the needle will respec-
tively richen or lean the mixture. At about 3/4 throttle the orifice
formed by the needle & 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 throt-
tle the needle has no influence.
ACCELERATOR PUMP
Mikuni's HSR42 carburetor is fitted with an accelerator pump.
The purpose of the pump is to inject fuel into the throat of the
HSR42 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. The pump maintains a
more correct mixture until the air velocity returns to normal.
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 also be controlled with dif-
ferent sized pump nozzles. Total flow volume depends upon
the pump's beginning & end point adjustments.
STARTER SYSTEM
Mikuni's starter system takes the place of the crude choke
mechanism of the stock carburetor. It is actually a small helper
carburetor designed to supply a rich mixture for starting pur-
poses. The starter system only works when the choke knob is
pulled open & when the throttle is closed. If the throttle is
opened with the choke knob pulled out, air will cease flowing
through the starter system & it will stop delivering its rich mix-
ture 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.
The starter system has one replaceable component, the starter
jet. A larger starter jet makes the starter mixture richer, a small-
er jet makes it leaner. The #55 jet fitted to the HSR42 will suit
most installations. However, if you live in a warm climate, the
alternate #45 jet may perform better.
DETAILS
1). Throttle return spring adjustment:
The throttle return spring has 3 pre-load positions. The HSR42
is supplied with the spring in the stronger of the 3 positions.
You may wish to try a weaker spring position. However, in
some installations, the softer spring position may result in
erratic return to full idle.
2). Fuel float adjustment:
Fuel float level is critical to proper operation of any carburetor,
the HSR42 is no exception. Mikuni correctly adjusts the floats
during assembly but they may be accidentally bent out of
SHOVELHEAD/IRON HEAD XL
Installation Instructions