Figure 4-51. Mechanical Progressive Linkage Operation.

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Figure 4-50. Two Barrel Carburetor with Fixed Linkage.

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TM-9-8000 Principles of Automotive Vehicles Manual

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Figure 4-52. Vacuum Progressive Linkage Operation.

TM 9-8000

Figure 4-51. Mechanical Progressive Linkage Operation.

whose vacuum source Is the primary venturl. The

throttle valves provides too much venturl area for the

principle of operation Is that as engine speed Increases,

engine to handle at low speed. It will not be able to move

the vacuum In the primary venturi also Increases,

enough air through the venturis to properly draw fuel from

causing the diaphragm to pull the secondary throttle

the discharge tubes, causing a lean mixture.

valves open. There Is linkage between the primary and

secondary throttle shafts to limit the opening of the

(2) The secondary section of the carburetor Is

secondary throttle valve(s) In relation to the primary

not equipped with an accelerator pump system and will

throttle valves.

cause an engine hesitation at low speed. To correct this

deficiency, most carburetors with mechanical progressive

f. Secondary Air Valve (Fig. 4-53). Carburetors

linkage use a secondary air valve. A secondary air valve

fits into the secondary throat and serves to restrict airflow

equipped with mechanically operated secondary throttle

through the secondary venturi(s) until the engine Is at a

valves are subject to engine hesitation if the throttle

high enough speed to utilize them correctly. The valve Is

suddenly is opened all the way at low engine speeds for

actuated by a vacuum under it that pulls it open against

the following reasons:

spring or counterweight force.

(1) The opening of primary and secondary

TA233416

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