9D-5

MODELS 4M-4MC-4MV PAGE 12

secondary throttle valves are opened, engine manifold vacuum (low pressure) is applied directly beneath the air valves. Atmospheric pressure on top of the air valves, provided the engine has warmed the choke coil to release the air valve lockout lever (if used), forces the air valves open against spring tension and allows metered air to pass through the secondary bores of the carburetor.

On most models, accelerating wells are used to supply fuel immediately to the secondary bores. This prevents a momentary leanness until fuel begins to feed from the secondary discharge nozzles.

When the air valves begin to open, the upper edge of each valve passes the accelerating well ports (one for each bore). As the edge of the air valves pass the ports, they are exposed to manifold vacuum and immediately feed fuel from the accelerating wells located on each side of the float bowl chamber. Each accelerating well has a 'calibrated orifice which meters the fuel supplied to the well from the float chamber. Some models have the accelerating well ports located beneath the front edge of the air valve instead of above. These begin to feed fuel to the secondary bores almost instantly after the secondary throttle valves open and before the air valves begin to open. This type porting is used on some models where added enrichment is needed during cold operation when the air valve is locked closed and also provides an earlier cut in of fuel from the ports than the models which have the port located above the valves. The use of either type of porting is dependent upon engine fuel demands.

The secondary main discharge nozzles (one for each bore) are located just below the center of the air valves and above the secondary throttle valves. The nozzles, being located in a low pressure area, feed fuel as follows:

As the secondary throttle valves are opened, atmospheric pressure opens the air valves. This rotates a plastic eccentric cam attached to the center of the air valve shaft. As the cam rotates, if lifts the secondary metering rods out of the secondary orifice plates through the metering rod lever which follows rotation of the eccentric cam.

Fuel flows from the float chamber through the secondary metering orifice plates into the secondary main wells where it is mixed with air from the secondary main well air bleed tubes. The air emulsified fuel mixture travels from the main wells through the secondary discharge nozzles where it sprays into the secondary bores. Here the fuel is mixed with air traveling through the secondary bores to supplement the air/fuel mixture delivered from the primary bores and goes on into the engine as a combustible mixture.

As the throttle valves are opened further and engine speeds increase, air flow through the secondary side increases and opens the air valves to a greater degree which in turn lifts the secondary metering rods further out of the orifice plates. The metering rods are tapered so that fuel flow through the secondary metering orifice plates is directly proportional to air flow through the secondary

carburetor bores. In this manner, correct air/fuel mixtures through the secondary bores are controlled by the depth of the metering rods in the orifice plates.

The depth of the metering rods in the orifice plates in relation to the air valve position are factory adjusted to meet air/fuel requirements for each specific engine model.

METERING RODS - PRIMARY

There are two types of primary main metering rods used in the Quadrajet carburetors. 1968 models use a rod which has a double taper at the metering tip. The 1967 and earlier models have a single taper at the metering tip.

Both type rods use 'a similar two-digit numbering system. The number indicates the diameter of the metering rod and is the last two digits of the part number. The 1968 models with the double taper will have "B" stamped on the rod after the two-digit number.

METERING RODS -SECONDARY

The secondary rods are coded with a two-letter system which corresponds directly to the part number.

(See Delco Bulletin 9A-100 for a complete description and listing of metering rods.)

There are other features incorporated in the secondary metering system as follows:

1. The secondary main well air bleed tubes extend downward into the main fuel well below normal fuel level. These bleed air into the fuel in the secondary wells to quickly emulsify the fuel with air for good atomization and improved fuel flow from the secondary nozzles.

2. The secondary metering rods have a milled slot at the larger diameter of the metering tip. The purpose of the slots is to ensure an adequate supply of fuel in the secondary main wells when the air valves are in the closed position. At this point, the metering rods are nearly seated against the metering orifice plates. The slot in the rod is adjacent to the orifice plate and allows a small amount of fuel to pass between the metering rod and metering disc. During extreme hot engine idle or hot soak the fuel could boil out of the secondary fuel wells. The milled slot allows enough fuel to by-pass the orifice plate and keep the main fuel wells full of fuel. This insures adequate fuel supply in the main wells at all times to give immediate fuel delivery from the secondary discharge nozzles. On some later Quadrajet models, the milled slot in the secondary metering rods is removed. Hot engine idle and hot soak tests have proved the slot is not needed on these applications.

3. There are two baffle plates in each secondary bore located just below the air valves. They extend up and around the secondary discharge nozzles. Their pur-