
03-69
SECTION 03 - ENGINE PREPARATION
Low Speed Screw
Low speed screw
is used in conjunction with the
idle stop screw to maintain idle speed and
smoothness. Use the low speed screw to adjust
the smoothness of the idle. Experiment turning
the low speed screw in and out in small incre-
ments until a smooth idle is obtained. As the idle
stop screw is turned in or out to raise or lower idle
speed, the low speed mixture is also affected.
The low speed screw is very sensitive and adjust-
ments should be made in small increments only.
For clarification, if the idle stop screw is turned
out to lower idle speed, the low speed air/fuel
mixture will richen slightly and a small adjustment
will need to be made with the low speed screw
inward to compensate.
NOTE:
To get a good “feel” for the correct low
speed adjustment, experiment with the idle stop
and low speed screws. You will find that a small
adjustment with one of the screws requires and
equal readjustment of the other.
Correct low
speed adjustment will result in improved throt-
tle response.
Transition Circuit
Transition circuit
is made up of the low speed
screw, pilot jet, bypass holes and pop-off pres-
sure. To adjust the transition circuit, change the
pilot jet or pop-off pressure. If a slight lean or rich
condition exists while accelerating from just off
idle to half throttle, change the pilot jet. Increasing
the jet size will richen the mixture. Make any jet
changes in small increments of jet sizes. If a se-
verely lean or rich condition exists in transition, it
will be necessary to readjust the pop-off pressure.
Refer to the pop-off chart to determine how to
change components for pop-off adjustment.
NOTE:
To accurately check and adjust pop-off
pressure, it will be necessary to use a pump
gauge tester (P/N 295 000 114), which is available
through your dealer. The pop-off chart in this sec-
tion is for approximating pressure only.
High Speed Screw
High speed screw
adjusts the air/fuel mixture, in
conjunction with the main jet, for maximum full
throttle performance. The initial setting for the
high speed screw is closed. If you are uncertain
that this setting will provide enough fuel, do not
hesitate to start with the screw out more turns.
Turning the screw clockwise (inward) will lean the
mixture, counterclockwise will richen the mixture.
The high speed screw is effective up to 3 full
turns out. The plastic cap must be removed for
adjustments over 1/4 turn.
Manifold Pressure
Manifold pressure is the
suction
generated by
the engine against a restriction; the strength of
this suction is measured in inches of mercury. In
the case of the Mikuni BN carburetor, the main
restriction is considered to be the throttle plate.
When the throttle plate is closed, it offers the
greatest amount of restriction and manifold pres-
sure is high. As the throttle plate is opened, the
restriction is reduced and manifold pressure de-
creases.
The carburetor sees manifold pressure as a po-
tentially strong suction drawing on any exposed
openings within the carb. As the throttle is
opened, more internal openings are exposed to
this suction. To begin with, only the low speed
fuel outlet is exposed, but as the throttle is
opened further, the bypass holes and the high
speed fuel from the inner venturi are exposed to
the suction. At this point, with the throttle wide
open, if there was no fuel in the carb, the engine
would not rev up and because the throttle plate
was open fully, the manifold pressure would be
very low. However, we do have fuel and the en-
gine does rev, drawing an increasing amount of air
velocity through the carb. As the air velocity
through the carb increases, it replaces manifold
pressure as the
suction
force within the carbure-
tor.
What are the dynamics of manifold pressure and
air velocity? As the engine is idling we know that
manifold pressure is high and there is good signal
strength within the carb, which makes adjust-
ment easy. As the throttle is opened quickly, man-
ifold pressure disappears and if the jetting within
the carb is correct, the engine revs quickly and air
velocity increases to accelerate you on your way.
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