and 3/4 throttle opening. Some mixture adjustment can be
accomplished by changing the location of the “E” clip on the
needle. Moving the clip down raises the needle in the jet
passage and richens the mixture. Moving the clip up lowers
the needle in the jet passage and leans the mixture. Letter and
number codes are stamped into the needle and the jet
indicating sizes and tapers of each.
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.
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