4.10
FUEL SYSTEM / CARBURETION
PILOT SCREW
The pilot system supplies fuel during engine operation
with the throttle valve closed or slightly opened. The fuel/
air mixture is metered by pilot screw and discharged into
the main bore through the pilot outlet.
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 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.
CAUTION
The pilot screw is calibrated at the factory to meet
EPA / CARB regulations for air quality standards
and is sealed with a brass plug to prevent tamper-
ing. Removal of the tamper proof plug is not permit-
ted. For service purposes, cleaning of the pilot
circuit can be done only by a certified repair shop to
ensure air quality standards are not exceeded.
Pilot Screw Location
Brass Plug Installed
Brass Plug Removed
E-Clip Position vs. Fuel Flow
Throttle Valve Opening
2
3
4
4
3
2
15
50
75
100%
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