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4.35
CARBURETOR - SPORTSMAN 450
4
CV Carburetor System Function Carburetor Venting
The carburetor float bowl vent lines supply atmospheric
pressure to the float bowl. The lines must be free of kinks,
restrictions and be properly routed. This allows fuel to flow in
the proper amount and prevents contaminants from entering the
carburetor.
Mikuni CV Carb Operation
The constant velocity carburetor incorporates a mechanically
operated throttle plate and a vacuum controlled slide valve
(vacuum slide). The venturi cross–sectional area in the
carburetor bore is increased or decreased automatically by the
vacuum slide, which moves according to the amount of negative
pressure (less than atmospheric) present in the venturi.
A diaphragm attached to the top of the vacuum slide is sealed to
the slide and to the carburetor body forming two chambers. The
chamber above the diaphragm is connected to the venturi area
by a drilled orifice in the center of the vacuum slide. The
chamber below the diaphragm is vented to atmospheric pressure
by a passage on the air box side of the carburetor. A spring,
installed in the center of the vacuum slide, dampens the slide
movement and assists the return of the slide.
When the throttle plate is opened and engine speed begins to
increase, the pressure in the venturi (and therefore in the
chamber above the diaphragm) becomes significantly lower
than atmospheric. Atmospheric pressure in the chamber below
the diaphragm forces the diaphragm upward, raising the slide
against spring pressure.
Carburetor Component Function
System
Main
Components
Main
Function
Main
Affect
Float
System
(Level
Control)
Inlet Pipe,
Needle and
Seat, Float,
Float Pin
Maintains
specified fuel
level in float
chamber
(carburetor float
bowl)
All systems
All throttle
ranges
Venting Passages in
Carburetor, Vent
lines
to frame
Supplies
atmospheric
pressure to float
chamber
All systems
All throttle
ranges
Starter
(Choke/
Enrichment)
Choke Lever,
Cable, Plunger,
Return Spring,
Carb Passages
(Starter Jet,
Starter Bleed
Pipe)
Supplies
additional fuel
air mixture
necessary for
cold starting
All throttle
ranges
Greatest
effect at low
throttle
settings and
idle
Pilot (Idle
System)
Pilot Jet/
Passage-ways,
Pilot–Mixture
Screw with
Spring Washer
and Sealing O–
Ring, By-pass
Ports (Behind
Throttle Plate),
Pilot Air Jet, Pilot
Outlet, Throttle
Plate
Primarily
supplies fuel at
idle and low
throttle positions
Mainly idle
to 1/4
throttle
Minimal
effect after
1/2 throttle
Main
System
Main Jet, Main
Air Jet, Main Air
Passage,
Needle Jet, Jet
Needle, Vacuum
Slide, Throttle
Plate
Supplies fuel at
mid–range and
high throttle
settings.
1/4 to full
throttle
Throttle Plate
Diaphragm
Low Pressure
From Venturi
Atmospheric
Pressure
Air inlet
Atmospheric
Va cu um
Slide
Ve nt ur i
= Air Flow
= Low pressure