Intake Tubes; Carburetor; Carburetor Operation - Yamaha MM700A Manual

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INTAKE TUBES

The carburetor is supported by a rubber intake tube
(Figure 2, shown with carburetor removed). Periodically
inspect the tubes for cracks or other damage which may
cause an air leak, causing a lean air/fuel mixture.

CARBURETOR

All engines are equipped with slide-valve type Mikuni
carburetor specifications and models.
A hand-operated choke control lever on the control
panel operates the starting enrichment system in the car-
buretor for cold engine starting. Fuel is supplied by a re-
mote pulse type fuel pump.

CARBURETOR OPERATION

For proper operation, a gasoline engine must be sup-
plied with fuel and air mixed in proper proportions by
weight. A mixture in which there is an excess of fuel is
said to be rich. A lean mixture is one that contains insufli-
cient fuel. A properly adjusted carburetor supplies the
proper mixture to the engine under all operating condi-
t ions.
The carburetors installed on all models consist of sev-
eral major systems. A float and fuel valve mechanism
maintain a constant fuel level in the float bowl. The pilot
system supplies fuel at low speeds. The main fuel system
supplies fuel at medium and high speeds. Finally, a primer
system supplies a rich mixture needed to start a cold en-
gine.
Float Mechanism
To ensure a steady supply of fucl, the carburetor is
equipped with a fuel inlet valve through which fucl flows
into the float bowl (Figure 3). Inside the bowl is a com-
bined float assembly that moves up and down with the
fuel level. Resting on the float arm is a fuel inlet valve,
which rides inside the fuel inlet valve seat. The fuel valve
regulates fucl flow into the float bowl. The fuel inlet valve
and inlet valve seat contact surfaces are accurately ma-
chined to ensure correct fuel flow calibration. As the float
rises, the fuel inlet valve rises inside the inlet valve seat
and blocks it, so when the fuel has reached the required
level in the float bowl, no more fuel can enter.
Pilot and Main Fuel Systems
The carburetor's purpose is to supply and atomize fuel
and mix it in correct proportions with air that is drawn in
through the air intake. At primary throttle openings (from
idle to 1/8 throttle) a small amount of fuel is siphoned
through the pilot let by suction from the incoming air
CHAPTER FIVE