Other 4MV models have the choke closing assist spring located on the vacuum break plunger stem replacing the torsion spring located on the intermediate choke shaft (Figure 30).
The choke closing assist spring aids in closing the choke valve along with tension from the remote choke thermostatic coil for improved engine starting. The choke closing assists spring only exerts pressure on the vacuum break link to assist in closing the choke valve during engine starting. When the engine starts and the choke vacuum break diaphragm seats, the closing spring retainer hits a stop on the plunger stem and the assist spring no longer exerts pressure on the choke valve.
The vacuum break diaphragm plunger is slotted to allow for free travel of the air valve dashpot link. The slot is moved from the air valve shaft lever to the vacuum break plunger. A change in air valve dashpot adjustment procedure is required. (See Adjustment Procedures in the 9D-5 Section, Delco Carburetor Parts and Service Manual 9X).
CHOKE SYSTEM WITH DELAYED VACUUM BREAK FEATURE
To delay the choke valve from opening too fast, some 4MV models use a delayed vacuum break system. An internal delay valve is included inside the diaphragm unit (Figure 31).
The delay feature operates as follows:
When the engine starts, vacuum acting on the internal delay valve bleeds air through a small hole in the valve which allows the vacuum diaphragm plunger to move slowly inward. This gives sufficient time to overcome engine friction and wet the engine manifold to prevent a lean stall. When the vacuum break diaphragm is fully seated, which takes a few seconds, the choke valve will remain in the vacuum break position until the engine begins to warm and relax the thermostatic coil located on the exhaust crossover in the intake manifold.
In addition to the internal delay valve, some car applications have a separate vacuum delay tank added to the system. This is connected "in series" to a second vacuum tube on the vacuum break diaphragm operation.
The delay valve in the choke vacuum diaphragm unit is designed to "pop" off its seat and allow the diaphragm plunger to extend outward, when the spring force against the diaphragm is greater than the vacuum pull. This will give added enrichment as needed on heavy acceleration during cold drive-a-way by allowing the choke coil to slightly close the choke valve. Some 4MV models use a calibrated restriction in the vacuum inlet to the vacuum break diaphragm unit in place of the internal delay valve. Similar to the internal delay valve, the calibrated restriction delays the supply of vacuum to the diaphragm unit to retard opening of the choke valve for good engine starting.
CHOKE SYSTEM WITH VACUUM BUCKING SPRING
A spring-loaded plunger is used in the vacuum break unit on some 4MV models (Figure 32). The purpose of the spring, called a "bucking spring", is to offset choke thermostatic coil tension and balance the opening of the choke valve with tension of the choke coil. This enables further refinement of air/fuel mixtures because the coil, which senses engine and ambient temperatures, will allow the choke valve to open gradually against spring tension in the diaphragm plunger head. In other words, in very cold temperatures, the extra tension created by the thermostatic coil will overcome the tension of the diaphragm plunger (bucking) spring to provide less choke valve opening with the result of slightly richer mixtures. In warmer temperatures, the thermostatic coil will have less tension and, consequently, will not compress the spring as much thereby giving a greater choke valve opening for slightly leaner mixtures.