4.9
FUEL SYSTEM / CARBURETION
MAIN SYSTEM
As throttle valve (1) is opened, engine speed rises, and
this increases negative pressure in the venturi.
Consequently the vacuum slide (2) moves upward. The
fuel in float chamber (3) is metered by main jet (4), and
the metered fuel enters needle jet (5), in which it mixes
with the air admitted through main air jet (6) to form an
emulsion. The emulsified fuel then passes through the
clearance between needle jet (5) and jet needle (7), and
is discharged into the venturi (A). Mixture proportioning is
accomplished in needle jet (5); the clearance through
which the emulsified fuel must flow is determined
ultimately by throttle position and vacuum slide height.
FLOAT SYSTEM
Fuel enters the float chamber (3) by means of the inlet
pipe and passage, through a screen on the back of the
inlet needle seat (4), and around the inlet needle (2). As
the fuel fills the float chamber, the float (1) rises and
forces the inlet needle against the seat, shutting off the
orifice in the seat. When fuel level is up in float chamber,
floats are up and needle valve remains pushed up against
valve seat. Under this condition, no fuel enters the float
chamber. As the fuel level falls, floats go down and
needle valve unseats itself to allow fuel into the chamber.
In this manner, the needle valve releases and shuts off
fuel alternately to maintain a constant fuel level inside the
float chamber.
1
2
3
4
5
6
7
A
Inlet
Pipe
1
2
3
4
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