CARBURETION
3.11
Slide Cutaway (1/8-3/8 Throttle)
Throttle valve cutaway effect is most noticeable at 1/4 throttle opening. The amount of cutaway is pre-determined
for a given engine to maintain a 14:1 air/fuel ratio at part throttle. A steep angle would indicate a fairly lean mixture
because there is less resistance to air flow. A flat angle would provide a much richer mixture because there is
more resistance to air flow. The venturi shape can be adjusted for each engine’s breathing characteristics by
using a different valve cutaway angle. A number will be stamped into the bottom of the valve (e.g. 2.5) indicating
the size of the cutaway. The higher the number, the steeper the angle. (Leaner mixture).
Throttle Valve
The throttle valve controls the rate of engine air intake
by moving up and down inside the main bore. At small
throttle openings, air flow control is performed chiefly
by the cutaway. By controlling air flow the negative
pressure over the needle valve is regulated, in turn
varying the fuel flow.
The throttle valves are numbered 1.0, 1.5, 2.0, etc.,
according to the size of the cutaway. The higher the
number, the leaner the gasoline/air mixture.
Air Jet
Throttle Valve
Jet Needle
By-pass
Pilot Outlet
Pilot Jet
Needle Jet
Main Jet
Cutaway Angle
Leaner
Richer
3.0
1.5
Air Flow
Cutaway
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