FUEL SYSTEM/CARBURETION
3.9
MAIN SYSTEM (3/4 TO FULL THROTTLE)
The main system is designed for delivering 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.
Air Jet
Throttle Valve
Jet Needle
By-pass
Pilot Outlet
Pilot Jet
Needle Jet
Main Jet
Air Box Side
Engine Side
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