4.5
CARBURETION
opening. Whether idling or at full throttle, the pressure built up
in the crankcase has enough amplitude to operate the pump.
When the piston is on the upstroke, crankcase pressure in that
cylinder becomes less positive. The diaphragm in the fuel
pump moves toward the engine, causing a negative pressure or
suction in the pump chamber. This causes the inlet valve from
the fuel supply to open and permits fuel to enter the chamber.
This same suction causes the outlet valve (to the carburetor) to
close so that fuel cannot return form the carburetor.
When the piston begins its downward stroke, the pressure
from the crankcase becomes positive, causing the fuel pump
diaphragm to move in the opposite direction and reversing the
pressure in the fuel pump chamber. This causes the inlet valve
in the pump to close and the outlet valve to open, filling the
float bowl in the carburetor. When the float level in the
carburetor reaches its standard level, the needle valve will
close, preventing more fuel from entering the carburetor, even
though the fuel pump continues to try to provide the carburetor
with fuel.
MAINTENANCE
The impulse operated diaphragm fuel pump does not require
any specific scheduled maintenance. However, the following
procedures should be observed.
OPERATION
The pump may be checked for operation by removing the fuel
supply line from the carburetor and placing it into a container.
With the engine idling at approximately 2000 RPM, a steady
flow of fuel should be visible.
CLEANING
The pump and impulse line must be disassembled and cleaned
of foreign material in the event of piston or other internal
engine part failures which produce fragments.
INSPECTION
Disconnect impulse line from pump. Connect a Mity Vac to
impulse fitting (or line) and apply 4-6 PSI pressure.
Diaphragm should hold pressure indefinitely.
The diaphragms and check valves must be carefully examined
for cracks, holes, or other damage. If in doubt as to the
condition of any internal parts, replace all diaphragms, check
valves, and gaskets.
MIKUNI JET NEEDLE
OVERVIEW
This needle (example) is a
9DH01-57. The first number
is the approximate overall
length in 10mm increments of
the jet needle. The 9 indicates
the needle is approximately
90mm but less that 100mm in
length.
The letters on the jet needle
indicate the angle of both
tapers. The first letter
designates the taper angle of
the top section (closest to the
grooves) and the second letter
designates the angle of the
bottom taper. The taper
angles are graduated in 15'
(15 minute) increments. The
jet needle marked 9DH01-57
would have a top taper of 1_0'
and a bottom taper of 2_0'.
The number following the
letters on the jet needle is the
serial number and it varies
with individual jet needles.
The last number, 57 indicates
that the outside diameter is
2.57mm. The smaller the O.D., the richer the mixture.
Table 4-1: Mikuni Jet Needle
DESIGNATOR DESCRIPTION
9 Overall length in 10mm increments
D Taper of the top section of the needle
H
taper of the bottom section of the
needle
01 Serial number
-57
Outside diameter (O.D.) of the
straight portion
To Next Page
Loading...
Need help?
General
