AIR / FUEL MIXTURE RATIO
A carburetor with a slide type throttle valve is also called a
variable venturi type carburetor. In this type of carburetor, the
needle jet and jet needle serve to control proper air/fuel
mixture ratio at the medium throttle valve opening (between
1/4 and 3/4 opening).
Having the proper needle jet and jet needle has a major impact
on engine performance at partial load. The jet needle tapers off
at one end and the clearance between the jet needle and the
needle jet increase as the throttle valve opening gets wider.
The air/fuel mixture ratio is auto controlled by the diaphragm
of vacuum chamber.
JET NEEDLE
The jet needle has tapered from approximately the middle of
the needle to the lower end. The top is fixed to the center of the
throttle valve by the needle clip, and the tapered end extends
into the needle jet.
Fuel flows through the space between the needle jet and jet
needle. This space does not vary until the throttle reaches the
1/4 open point. At that time the tapered portion of the needle
begins to move out of the jet, affecting fuel flow as the
opening enlarges. If the needle clip is changed from the
standard position to a lower groove, the needle taper starts
coming out of the jet sooner, resulting in a richer mixture.
Moving the clip higher produces a leaner mixture. If the taper
is worn due to vibration, fuel flow may be significantly
affected.
NEEDLE JET
The needle jet works in conjunction with the jet needle to
regulate fuel flow rate. An air bleed opening in the side of the
needle jet brings in air measured by the air jet. This air initiates
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