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AC3 Series of Engines
32
Carburetor
The AC3 series of engines uses the Walbro WYL series carburetors, or a carburetor of similar design from
another manufacturer.
The main distinguishing features of this carburetor design are:
• It has a rotary throttle valve, not a butterfly valve.
• A fuel metering needle in the carburetor venturi rises out of an orifice in direct proportion to the movement
of the throttle valve.
This design allows fuel to be metered into the venturi
in amounts that are directly related to the amount of air
allowed to pass through the venturi. The air flow is regu-
lated by the rotary throttle valve.
The Rotary valve resembles a round drum with a hole
bored through it. See Figure 5.8.
• When the hole through the drum is aligned with
the throat of the carburetor, the throttle is
opened.
• When the drum is rotated so that the hole is not
aligned with the throat of the carburetor, the
throttle is closed.
• A groove around the drum allows some air to
pass-by the drum when the throttle is closed,
providing the engine with enough air to idle.
The following picture sequence illustrates the opening
of the rotary valve throttle:
1. In the first frame, the throttle is fully closed (idling or
stopped). See Figure 5.9.
Figure 5.8
Throttle assembly
Fuel metering needle
Idle groove
Orifice
Figure 5.9