Figure 4-48. Fast Idle Cam Operation.
common choke, float, and accelerator pump system
(2) The operating range of the engine can be
increased and drivability improved if the throttle linkage is
b. Purpose. There are two reasons for using
arranged to phase in the carburetor venturis gradually as
the accelerator pedal is depressed.
There are two basic arrangements for the throttle
(1) The use of two separate carburetors, each
linkage, depending on the purpose for having multiple-
feeding separate cylinders, can help to improve fuel
The linkage arrangements are
discussed in the next two subparagraphs.
c. Fixed Throttle Linkage (Fig. 4-50). Fixed throttle
linkage is used mostly on two-venturi or two-barrel
carburetors on engines containing six or more cylinders.
This linkage arrangement usually is installed on an intake
manifold that is arranged so that each venturi is feeding a
selected half of the cylinders. The separation of the
carburetor venturis within the intake manifold usually is to
keep consecutively operating cylinders separated.
Whenever two cylinders go through power strokes
consecutively, the second of the two cylinders tends to
have its fuel supply cut off. The fixed throttle linkage
arrangement is a solid throttle shaft that operates both
throttle valves simultaneously. There is an idle mixture
screw on each side of the carburetor. The accelerator
pump discharge nozzle usually contains two outlets.
d. Progressive Throttle Linkage. Progressive throttle
linkage is set up to open one throttle valve or one set of
throttle valves at the beginning of the linkage travel and
begin to open the second throttle valve or set
Figure 4-49. Choke Unloader