CARBURETION
3.12
Jet Needle/Needle Jet (3/8-3/4 Throttle)
The jet needle and needle jet have the most effect be-
tween 3/8 and 3/4 throttle opening. Some mixture adjust-
ment can be accomplished by changing the location of the
“E” clip on the needle. Moving the clip down raises the
needle in the jet passage and richens the mixture. Moving
the clip up lowers the needle in the jet passage and leans
the mixture. Letter and number codes are stamped into
the needle and the jet indicating sizes and tapers (needles
only) of each.
Jet Needle / Needle Jet - Fig. 1
The jet needle tapers off at one end and the clearance be-
tween the jet needle and the needle jet increases as the
throttle valve opening gets wider. The air/fuel mixture ratio
is controlled by the height of the “E” ring inserted into one
of the five slots provided in the head of the jet needle. The
chart at right shows the variation of fuel flow based on the
height of the “E” ring.
Needle Jet - Fig. 2
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 - Fig. 3
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.
Small
Clearance
Medium
Clearance
Large
Clearance
Closed
Throttle
One-half
Throttle
Full
Throttle
Fig. 3
Air Jet
Throttle Valve
Jet Needle
By-pass
Pilot Outlet
Pilot Jet
Needle Jet
Main Jet
Clip Position
2
3
4
4
3
2
15
50 75 100%
Throttle Valve Opening
Fig. 1
Fig. 2
Air Bleed
Opening
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