Carbureted Models Only
See Figure 1
To reduce emissions, carburetor air/fuel mixtures are calibrated to be as lean as possible. Although a lean mixture will burn readily at hotter temperatures, it is reluctant to ignite when cold, not because of the mixture as such, but because fuel vaporizes less readily when cold. Thus, increasing the amount of fuel in a cold air/fuel mixture increased the amount of vaporized fuel available.
The problem of a poor air/vaporized fuel mixture in a cold engine is accentuated when accelerating. Although the carburetor is equipped with an acceleration pump for normal accelerating engine demands, its capacity is insufficient for a cold engine. The auxiliary acceleration pump (AAP) is designed to send additional fuel into the acceleration nozzle in the carburetor independent of the regular acceleration pump.
The AAP itself is an integral part of the carburetor. It consists of two check valves controlled be springs, and a diaphragm controlled by both a spring and engine vacuum obtained from the intake manifold. At constant speeds, intake manifold vacuum draws the AAP DIAPHRAGM BACK, ENLARGING THE AAP CHAMBER AND THUS ALLOWING GASOLINE TO ENTER. When the engine is accelerated, intake manifold vacuum drops, allowing the AAP DIAPHRAGM TO BE PUSHED BACK BY THE SPRING. The resultant reduction in chamber volume forces the gasoline out through the other check valve, into the acceleration nozzle. Thus, the engine gets a needed squirt of gasoline.
AAP operation is governed by the same TVSV described in the AI section earlier. At cold coolant temperatures, the TSVS allows intake manifold vacuum to reach the AAP diaphragm. At approximately 122°F, the TSVS closes off the passage to vacuum, thus shutting off the AAP.
See Figure 2
The vacuum hose should be checked for leaks, kinks, or improper connection.
- With the engine cold (below 75°F) and idling (front wheels blocked, parking brake on, transmission in neutral) remove the air cleaner cap and look into the carburetor. At the instant the vacuum hose is removed from the AAP, gasoline should squirt from the nozzle.
- If it does not, check for vacuum in the hose. If present, the AAP diaphragm may be defective, the nozzle may be blocked, the check valves may be stuck, or gasoline may not be flowing into the chamber.
- If there is no vacuum in the line, either the line has an air leak or the TSVS is defective.
- After warming the engine to operating temperature, perform the same test as in Step 1. If gasoline spurts out, the TSVS is defective.