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5.4
Section 5
Fuel System and Governor
Slow Circuit: (Figure 5-2) At low speeds the engine operates only on the slow circuit. As a metered amount of air
is drawn through the slow air bleed jet, fuel is drawn through the main jet and further metered through the slow jet.
Air and fuel are mixed in the body of the slow jet and exit to the transfer port. From the transfer port this air fuel
mixture is delivered to the idle progression chamber. From the idle progression chamber the air fuel mixture is
metered through the idle port passage. At low idle when the vacuum signal is weak, the air fuel mixture is
controlled by the metered idle fuel passage. This mixture is then mixed with the main body of air and delivered to
the engine. As the throttle plate opening increases, greater amounts of air fuel mixture are drawn in through the
fixed and metered idle progression holes. As the throttle plate opens further the vacuum signal becomes great
enough so the main circuit begins to work.
Figure 5-2. Slow Circuit.
Fuel Inlet
Float Valve Seat
Float Valve
Main Jet
Float
Main Emulsion
Hole
Idle Speed
(RPM)
Adjustment
Screw
Air
Fuel
Mixture
Check Valve Spring
Outlet Check Valve
Leak Jet
Adjustment Screw
Diaphragm Spring
Pump Diaphragm
Inlet Check Valve
ACCELERATOR PUMP
ASSEMBLY
(Some Carburetors)
Fuel Shut-Off Solenoid
with Main Jet
Main Jet
Slow Passage Pipe
Main Nozzle
Throttle Valve
Idle
Progression
Holes
Idle Limiter
Jet
Capped/Preset
Low (Idle)
Mixture Setting
Idle Port
Idle Progression
Chamber
Slow
Jet
Slow Air Bleed Jet
Choke Valve
Main Air Bleed Jet
Accelerator Pump Nozzle
Bowl Vent