Jet Needle / Needle Jet / Throttle Opening - Polaris SCRAMBLER 50 Service Manual

AIR SCREW
The air screw controls the fuel mixture from idle to low
speeds. The tapered tip of the air screw projects into
the air passage leading to the pilot jet air bleeds. By
turning the screw in or out, the cross sectional area of
the air passage is varied, in turn varying the pilot jet
air supply and changing the mixture ratio.
Air Screw
AIR/FUEL MIXTURE RATIO
E-Clip Position vs. Fuel Flow
2
3
4
4
15
Throttle Valve Opening
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
3.6
3
2
50 75 100%
FUEL SYSTEM/CARBURETION
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 increases
as the throttle valve opening gets wider. The air/fuel
mixture ratio is controlled by the height of the "E" clip
inserted into one of the five slots provided in the head
of the jet needle. The previous chart shows the
variation of fuel flow based on the height of the "E" clip.
JET NEEDLE
The jet needle has five adjustment grooves cut into
the upper 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 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 the mixing and atomizing process
Jet Needle
Leaner
Groove
Richer