Fuel Metering; Fuel Delivery; Pilot Jet; Pilot Air Screw - Polaris 340 CLASSIC 2006 Manual

CARBURETION

FUEL METERING

Mikuni carburetors use a starter enricher system rather than a
choke. In this type of carburetor, fuel and air for starting the
engine are metered with entirely independent jets. The fuel
metered in the starter jet is mixed with air and is broken into
tiny particles in the emulsion tube. The mixture then flows
into the plunger area, mixes again with air coming from the air
intake port for starting and is delivered to the engine through
the fuel discharge nozzle in the optimum air/fuel ratio. The
starter is opened and closed by means of the starter plunger.
The starter type carburetor is constructed to utilize the
negative pressure of the inlet pipe, so it is important that the
throttle valve is closed when starting the engine
FUEL
DELIVERY
The pilot system's
main function is to
meter fuel at idle
and low speed
driving. Though
its main function
is to supply fuel at
low speed, it does
feed fuel
continuously
throughout the
entire operating
range.
Fuel for the pilot
jet is drawn from
the float bowl, mixed with air regulated by the air screw, and
delivered to the engine through the pilot outlet.
4.8
FUEL METERING
The mixture is regulated to some degree by adjusting the air
screw. When the air screw is closed, the fuel mixture is made
richer as the amount of air is reduced. When the air screw is
opened, the mixture is made more lean as the amount of air is
increased.
The main system is designed to deliver fuel between low
speed and high speed operation. This system is made up of the
jet needle, needle jet, and main jet. The main system begins to
take effect as soon as there is enough air flow into the
carburetor venturi to draw fuel up through the main jet and
needle jet assembly. This system works in conjunction with
the needle jet system.
During low speed driving, there is very little clearance
between the jet needle and the needle jet; therefore, very little
fuel from the main jet can pass between the jet needle and the
needle jet. As the throttle valve opening is increased, the
tapered jet needle is raised farther out of the needle jet,
allowing greater fuel flow. Under full throttle opening, the
cross sectioned area of clearance between the jet needle and
the needle jet becomes greater than the cross sectioned area of
the main jet. Thus the main jet is now controlling the amount
of fuel flow.

PILOT JET

From idling to low speeds, the fuel supply is metered by the
pilot jet. There are several air bleed openings in the sides of
the pilot jet which reduce the fuel to mist. The number
stamped on the jet is an indication of the amount of fuel in cc's
which passes through the jet during a one minute interval
under a given set of conditions.

PILOT AIR SCREW

The pilot 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.

MAIN JET

When the throttle
opening becomes
greater and the area
between the needle jet
and jet needle
increases, fuel flow is
metered by the main
jet. The number on the jet indicates the amount of fuel cc's