Variable Venturi Carburetor Description; Fuel Input System - Ford VSG-411 Service Manual

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Fuel System
3-10
VARIABLE VENTURI CARBURETOR
DESCRIPTION
The variable venturi carburetor adjusts the size of the venturi to
match the engine' s air demand thereby retaining the high air intake
speed required for good fuel atomization. For example, at max-
imum engine speeds the air valve in the venturi is fully open
allowing sufficient air to enter the engine to produce maximum
power output. As the engine speed and power reduces, the air
required for the engine is also reduced. The air valve closes the
venturi which results in a high air speed similar to that achievedat
high engine revolutions being available throughout the whole
engine speed range.
Figure 5 shows the variable venturi carburetor in the low engine
speed condition. Note that the air valve has reduced the size of the
venturi resulting in a much higher air speed.
FIG. 5 Variable Venturi Carburetor at Low Engine
Speed and Load
A - Air intake
B - Air Valve (Almost Closed)
c - Throttle Plate
Control of the air supply on both Variable Venturi and Fixed
Venturi carburetors is achieved by a throttle butterfly plate
mounted in the venturi. It is in the method of controlling the fuel
that the types of carburetor differ.
On the fixed venturi unit the main system does not discharge
until the throttle plate is open and the increased air flow creates
sufficient vacuum to pull fuel through the main nozzle.
It is difficult to achieve a smooth transition from the idle to the
main system and at the point where the main jet system comes into
operation, fuel atomization is poor with large droplets of fuel
being fed into the engine. A good drive condition is achieved
during this transition by richening the mixture strength to compen-
sate for the poor fuel atomization.
The variable venturi carburetor does not have this disadvantage
because the main jet system, with its associated high velocity, is in
operation at all engine speeds and loads.
The reason fuel is drawn in through the main jet system on the
VV carburetor at low engine speeds and loads is that the vacuum at
the
main jet outlet is high, due to the air valve reducing the venturi
size and thereby increasing the air velocity.
The amount of fuel passed into the engine is controlled by a
tapered metering rod, attached to the venturi valve, which slides
through the main jet (Figure 6). As the engine' s fuel demand is
increased the tapered rod is pulled outwards through the main jet.
This enlarges the main jet size and therefore allows more fuel to be
fed through the system.
FIG. 6 VV Main Jet System
A - Tapered Metering Rod
B - Main and Secondary Jets
C- Main Fuel Outlet
OPERATION
The operation of the VV carburetor has been sub-divided into
nine sub-systems to help simplify and explain its various working
conditions.
l Fuel Input
l Venting System
l Air Control
l Main Fuel Control
l Idle System
l
Throttle Operation
l Accelerator Pump
l
Choke Operation
l
Anti-Dieseling Valve
FUEL INPUT SYSTEM
Fuel input is via an inlet filter, located in the inlet tube, a
conventional needle valve and a float located in the float chamber
(Figure 7).
FIG. 7 Fuel Input System
A - Inlet Filter
B - Needle Valve
c- Float Pivot
D- Float