vent connector tube leading to the vapor canister, to vent excessive fuel vapors that form in the fuel chamber above the fuel surface that could disrupt hot engine operation. A plastic filler block is located in the top of the fuel chamber over the float valve to prevent fuel slosh in this area. On M4MC and M4ME models, either a filler spool or an aneroid cavity insert is used in the fuel chamber to reduce fuel slosh on turns. Also, a metal baffle is added to the pump well fill slot in the float bowl of M4MC-M4ME models to reduce fuel slosh during various vehicle maneuvers.
On most models, an integral 1-inch or 2-inch pleated paper fuel inlet filter, dependent upon model, is mounted in the front of the float bowl behind the fuel inlet nut to filter impurities from the incoming fuel. If used, a check valve is pressed into the neck of the fuel filter. The check valve consists of a plastic disc contained in a plastic retainer. It is held in the normally closed position by a small spring which exerts pressure on the check valve. When the engine starts and fuel flow pressure from the fuel pump enters the inlet nut, it pushes the small check valve off its seat. Fuel flows past the valve into the inside of the filter and continues on through the filter to the float valve and seat. With the engine off, the check valve closes and shuts off fuel flow to the carburetor to prevent fuel leaks if a vehicle roll-over should occur.
The check valve retainer also has a flanged neck which seals between the filter and fuel inlet nut.
NOTICE: If used, the check valve must be installed to meet government safety standards for roll-over. New service replacement filters include the check valve where required.
The fuel filter is held in position by the force of a spring located between the filter assembly and the fuel inlet nut cavity.
NOTICE: It is very important that the filter be serviced according to recommended maintenance intervals to prevent dirt and other impurities from entering the carburetor metering orifices.
The float system operates in the following manner:
Fuel flow from the fuel pump enters the carburetor fuel inlet nut. It opens the check valve (if used) in the filter against spring force and flows through the filter element (or strainer), and then passes from the filter chamber up through the float valve seat and flows past the float valve on into the fuel chamber. As the incoming fuel enters the fuel chamber, the float pontoon rises and forces the float valve closed, shutting off fuel flow at a prescribed level. As fuel is used from the fuel chamber, the float pontoon drops to open the float valve allowing fuel to again enter the fuel chamber. This cycle continues, maintaining a near constant fuel level in the fuel chamber for all ranges of engine operation.
A float pull clip, fastened to the float valve, hooks over the edge of the float lever at the center as shown in Figures 6 or 7. Its purpose is to assist in lifting the float valve off its seat whenever fuel level in the fuel chamber is low.
NOTICE: Do not place pull clip through small holes in top of float lever. Severe flooding will result.
There are two types of float valves used in the Quadrajet carburetor: One type is diaphragm assisted and the other is the conventional needle and brass seat.
The diaphragm assisted float valve (shown in the inset Figure 6) is used primarily with a smaller float and on engines equipped with high pressure fuel pumps. The valve seat is a brass insert and is pressed into the bowl fuel inlet channel below the diaphragm needle tip. The seat is not removable, as the valve tip is of a material which makes seat wear negligible. Care should be used during servicing so that the seat is not nicked, scored, or moved. The float valve is factory staked and tested and should not be re-staked in the field.
Fuel flow through the diaphragm assisted float valve varies from the conventional float needle. With the conventional type (as shown in Figure 7) fuel flows from the inlet filter and inlet channel up through the needle seat orifice past the float needle valve and spills over into the float bowl. With the diaphragm type float valve, fuel from the inlet filter enters the channel above the float valve tip. When fuel level is low in the bowl, the float valve is off its seat and fuel flows down past the valve tip into a fuel channel which leads upward through the bowl casting to a point above normal liquid level and spills over into the float bowl.
The diaphragm type float valve differs in operation from the conventional float needle in that a larger seat orifice can be used to provide greater fuel flow to the float chamber and yet allow the use of a small float. This is accomplished through a balance of forces acting on the float valve and diaphragm against fuel pump pressure. Fuel pressure entering the float valve chamber tends to force the valve closed. However, the same pressure is also acting on the float valve diaphragm. The diaphragm has a slightly larger area than the float valve head; therefore the greater pressure acting on the diaphragm tends to push the valve off its seat. The force of the float arm acting on the valve stem, as the float bowl fills, overcomes this pressure difference and closes the valve. Therefore, the float’s function is to overcome the pressure difference and it does not have to force a valve closed against direct fuel pump pressure as does the conventional needle type.
To improve hot engine starting and to prevent rough idle which may result from excessive fuel vapors that may form in the fuel chamber, various methods have been used to vent these vapors external to the carburetor.
Early Quadrajet applications vented these vapors to the atmosphere through an idle vent valve that is operated mechanically. A wire tang on the pump lever opens the valve during idle and allows the valve to close at greater throttle valve angles as in the off idle and part throttle positions.
Some models use a thermostatically controlled vent valve (Figure 8). This heat sensitive valve is operated by a bi-metal strip which holds the valve closed at temperatures below 75F. When temperatures at the carburetor exceed that value the valve opens allowing vapors to escape. The thermostatic vent valve is adjustable to make sure it is timed to open during idle and closed at higher engine speeds.
Some Quadrajets have a vacuum operated vent valve (Figure 9) that is also controlled to function with an evaporative vapor canister. The vacuum vent valve is designed to be open and to allow fuel chamber vapors to be routed directly to a vapor collection canister instead of the atmosphere for improved evaporative emission control during engine shut-down. A passage beneath the vacuum diaphragm in the air horn provides a constant purge of the vapors from the vapor canister during off idle and higher engine speeds.
Later “modified” versions of the Quadrajet have a bowl vent valve that is spring-loaded and is actuated by action of a spacer on the pump plunger shaft (Figure 10). The fuel chamber is externally vented to the vapor collection canister during periods of engine shut-down.
Another method used to vent vapors is by the addition of a tube to the air horn (see Figure 7). Vapors flow through this tube and connecting hose to a vacuum operated vapor vent valve that is an integral part of the vapor collection canister located elsewhere in the engine compartment. The canister vent valve is spring-loaded and normally open, allowing bowl vapors to pass into the canister during engine shut down. Manifold vacuum during engine operation closes the valve and vapors are purged from the canister. This system improves hot engine starting and also meets government evaporative emission requirements.
NOTICE: External venting of fuel chamber vapors is not used on Quadrajet carburetors designed for marine use.
