PRINCIPLES GASOLINE/DIESEL FUEL SYSTEMS - OD1620
- LESSON 1/TASK 2
(4) 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 to begin to open the second throttle
valve, or set of throttle valves, when the first is about twothirds open.
The geometry of the linkage is set up so that as the throttle reaches the
full open point, all of the throttle valves will be wide open. The purpose
is to provide a carburetor that will have a venturi small enough to provide
good throttle response and fuel economy at low speed, yet large enough to
allow the engine to perform well at high speed. The section of the
carburetor that operates at low speed is called the primary section; the
section that operates at high speed is celled the secondary section. It
should be noted that the primary section of the carburetor works throughout
the engine's operational range. It should also be noted that the secondary
section of the carburetor has no choke, accelerator pump, lowspeed, idle,
or highspeed enrichment system. These systems are unnecessary in the
secondary section of the carburetor for the following reasons:
(a) The secondary section of the carburetor is locked out so that
its throttle valve(s) will not open when the choke system is operational on
the primary side of the carburetor.
(b) At the speed that the engine is operating when the secondary
section of the carburetor begins to operate, there will be no hesitation
(c) Because the secondary section of the carburetor only operates
at high speeds, it does not have to be jetted for two stages of operation
and therefore will not require a highspeed enrichment system.
(d) Because the secondary section of the carburetor operates only
at high speeds, a lowspeed and idle system are unnecessary.
There are numerous devices and systems of linkage that are used to make the
secondary section of the carburetor operate. They will be discussed in the