FUEL SYSTEM/CARBURETION
3.10
FUEL DELIVERY (2 CYCLE)
The throttle opening chart below demonstrates component relationship to fuel flow versus throttle valve opening.
The pilot system’s main function is that of a low speed jet. Its most effective range of fuel delivery is from idle
to approximately 3/8 throttle valve opening.
The throttle valve controls the rate of engine air by its movement up and down in the carburetor venturi. At small
throttle openings the air flow is regulated chiefly by the valve cutaway, with greatest effectiveness at 1/4 throttle
opening. Throttle valves are numbered 1.0, 1.5, 2.0, etc., according to the size of the cutaway. Decreasing the
cutaway number will increase the amount of fuel delivered in its effective range.
The jet needle and needle jet have an effective operating range from approximately 1/8 to 7/8 throttle opening.
The amount of fuel delivered during this range relies upon the jet needle clip position, as well as the needle jet
size and other specifications.
The main jet affects fuel delivery at 1/4 throttle and consistently increases to full throttle opening.
Carburetor Component Function - 2 Cycle
System Main Components Main Function Main Affect
Float System (Fuel Level
Control)
Inlet Pipe, Needle and Seat,
Floats, Float Pins
Maintains specified fuel lev-
el in float chamber (carbure-
tor float bowl)
All systems
All throttle ranges
Venting Vent Passages in Carbure-
tor, Vent line
Supplies atmospheric pres-
sure to fuel in float chamber
All systems
All throttle ranges
Starter (Choke/Enrichment) Choke Lever, Cable, Choke
Plunger, Return Spring,
Carb Passages (Starter Jet,
passage in float bowl)
Supplies additional fuel air
mixture necessary for cold
starting
All throttle ranges
Greatest effect at low
throttle settings and idle
speeds
Pilot (Idle System) Pilot Jet/Passageways, Pilot
Air Screw with Spring, By-
pass Port (Beneath Throttle
Slide), Air Jet, Pilot Outlet,
Throttle Valve Cutaway
Primarily supplies fuel at idle
and low throttle settings
Mainly idle to 1/4 throttle
Minimal effect after 1/2
throttle
Main System Main Jet, Main Air Passage,
Needle Jet, Jet Needle,
Throttle Valve
Supplies fuel at mid-range
and high throttle settings.
1/4 to full throttle
INCREASINGDECREASING
EFFECT
THROTTLE OPENING - 2 Cycle
Throttle Valve Cut-Away
Jet Needle/Needle Jet
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