FUEL SYSTEM/CARBURETION
3.4
JET NEEDLE
The jet needle has five adjustment grooves cut into the up-
per portion, and is 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 pro-
duces 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 the mixing and atomizing process inside the needle
jet. Mixing is augmented by a projection at the needle jet
outlet, called the primary choke. The letter number code
stamped on the jet indicates jet inside diameter.
THROTTLE OPENING VS.
FUEL FLOW
In a full throttle condition the cross sectioned area be-
tween the jet needle and the needle jet is larger than
the cross sectioned area of the main jet. The main jet
therefore has greater control over fuel flow.
Needle Jet
Opening faces
engine
Washer
Main Jet
Small
Clearance
Medium
Clearance
Large
Clearance
Closed
Throttle
One-half
Throttle
Full
Throttle
Groove
Leaner
Richer
Jet Needle
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