Carburetor Basics WM 80 Repair
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Partial Throttle:
At partial throttle, the throttle shutter is partially opened allowing a
greater amount of air through the carburetor. The throttle has opened
wide enough to expose the secondary-idle discharge port and the
engine suction draws fuel from both the primary-idle and secondary-
idle discharge ports.
Idle and Low Speed:
The amount of air flowing through the carburetor is regulated by the
throttle and choke shutters. At idle, only a small amount air flow is
needed. This air flow is regulated by the throttle shutter alone, because
the choke is open and no longer affects air flow. Idle speed can be
adjusted by the idle speed screw (e).
Start-up Mode (Choke):
During cold start-up, the choke shutter (butterfly) is closed and the
throttle shutter is partially open. As the engine is turned over, engine
suction draws fuel from the primary, secondary, and main fuel
discharge ports. Because the choke is closed, the amount of air
flowing through the carburetor is very restricted. As a result, a rich fuel
mixture (fuel mixed with a relatively small amount of air) is delivered to
the engine which allows the engine to start more easily.
Full Throttle:
As the throttle continues to open, the velocity of the air through the
carburetor venturi (f) increases and creates a low pressure area over
the main fuel discharge port (g). When the air pressure in the venturi
becomes lower than the pressure on the fuel in the fuel chamber
(h),
the fuel will flow out into the throat of the carburetor and mix with the
air stream. The fuel will also flow out of the primary and secondary
discharge ports. The wider the throttle is opened, the more air that will
flow through the carburetor. The more air flowing, the faster it will flow
over the venturi. The faster over the venturi, the lower the pressure
over the main discharge port and thus the more fuel which will flow out
into the air stream. This phenomenon allows the carburetor to maintain
the same approximate air/fuel ratio as the air flow varies.
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