Progressive Throttle Linkage

Principles of Gasoline And Disesel Fuel Systems
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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

which would make an acceleration pump system necessary.

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

following paragraphs.