Cv Carburetor System Function; Carburetor Venting; Mikuni Cv Carb Operation - Polaris 2007 Sportsman 500 EF Service Manual

CV Carburetor System Function

Carburetor Component Function
Main
System
Components
Float Inlet Pipe,
System Needle and
(Level Seat, Float,
Control) Float Pin
Venting Passages in
Carburetor, Vent
lines
to frame
Starter Choke Lever,
(Choke/ Cable, Plunger,
Enrichment) Return Spring,
Carb Passages
(Starter Jet,
Starter Bleed
Pipe)
Pilot (Idle Pilot Jet/
System) 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
Main Main Jet, Main
System Air Jet, Main Air
Passage,
Needle Jet, Jet
Needle, Vacuum
Slide, Throttle
Plate
Main Main
Function Affect
Maintains All systems
specified fuel All throttle
level in float ranges
chamber
(carburetor float
bowl)
Supplies All systems
atmospheric All throttle
pressure to float ranges
chamber
Supplies All throttle
additional fuel ranges
air mixture Greatest
necessary for effect at low
cold starting throttle
settings and
idle
Primarily Mainly idle
supplies fuel at to 1/4
idle and low throttle
throttle positions Minimal
effect after
1/2 throttle
Supplies fuel at 1/4 to full
mid–range and throttle
high throttle
settings.
CARBURETOR - SPORTSMAN 450

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.
Diaphragm
Throttle Plate
= Air Flow
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.
Low Pressure
From Venturi
Atmospheric
Pressure
Atmospheric
Air inlet
Vacuum
Slide
Venturi
= Low pressure
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