Unlike many 4 barrel carburetors, the Rochester Quadrajet uses a single pontoon float and a single float reservoir.
The Quadrajet carburetor is unique in that it has a centrally located fuel reservoir (float bowl). The float bowl is centered between the primary bores and is adjacent to the secondary bores. This type of design assures an adequate fuel supply to all carburetor bores, resulting in excellent performance with respect to car inclination or severity of turns.
The original float pontoons were solid and made of light, closed-cell plastic. This plastic will absorb fuel overtime and we recommend replacing them during a rebuild. Many of the Quadrajet floats are now offered in brass which is what we recommend when you can get them.
The float system consists of a fuel chamber or float bowl, a single pontoon float, float hinge pin and retainer combination, float needle and seat with pull clip, idle vent valve (some applications), vacuum vent switch valve (some applications), and internal vents. A plastic filler block is located in the top of the float chamber over the float valve to prevent fuel slosh into this area. The secondary sponge seal is available here.
On most models, a fuel inlet filter is an integral part of the float bowl, with the filter located behind the fuel inlet nut. Should the filter become plugged due to excessive dirt or improper service, a pressure relief spring located behind the filter element allows fuel pump pressure to force the element off its seat. This allows fuel to by-pass the filter and enter the carburetor so the engine will run until the filter can be serviced.
There are 4 different Quadrajet floats available:
Fuel from the engine fuel pump enters the carburetor fuel inlet passage. It passes through the filter element, fuel inlet valve, and on into the float bowl chamber. As the incoming fuel fills the float bowl to the prescribed level, the float pontoon rises and forces the fuel inlet valve closed, shutting off fuel flow. As fuel is used from the float bowl, the float drops allowing the float valve to open, allowing more fuel to again fill the bowl. This cycle continues, maintaining a constant fuel level in the float bowl.
A float needle pull clip, fastened to the float needle valve, hooks over the edge of the float arm at the center as shown in the above figure. It's purpose is to assist in lifting the float valve off it's seat whenever fuel lever in the bowl is low.
CAUTION: Do not place pull clip through small holes in top of float arm, flooding will occur.
The conventional float needle seat on some Quadrajet carburetors has holes or "side windows" on its side whose purpose is to supplement fuel delivery past the float needle valve when it opens, whereas the side windows have been removed on some float needle seat applications so that all fuel will be discharged over the top of the needle seat assembly. Remove of the side windows improves operation of the float needle on some applications. Some conventional float needle seats are nickel plated and use a double angle seat. The double angle seat, along with use of a less resilient viton tip float needle, aids in preventing the float needle from possibly sticking in the seat due to fuel gum formation.
The carburetor float chamber is internally vented on all models through a vent tube or tubes located in the air horn. The internal vent tube(s) leads from beneath the air cleaner to the float bowl chamber. The purpose is to balance air pressure acting on the fuel in the bowl with air flow through the carburetor bores. In this way, balanced air/fuel mixture ratios can be maintained throughout all carburetor ranges of operation.
Some engine applications require an external vent into the float bowl during hot engine operation. The Quadrajet float bowl is externally vented through an idle vent valve. Its purpose is to vent fuel vapors which may form in the float bowl during periods of hot engine idle and heat soak.
In operation, the idle vent valve is closed except when the throttle valves are in the idle position. When the throttles are closed, a wire tang on the pump lever pushes upward on the spring steel vent valve arm and opens the vent valve. Thus fuel vapors are allowed to vent externally, thereby preventing them from entering the carburetor bores and being drawn into the engine. This prevents rough engine idle and excessively long, hot engine starting. When the throttle valves are open to the off-idle and part throttle position, the idle vent valve closes, returning the carburetor to internally balanced venting.
A temperature controlled idle vent valve is used on some models. In place of the standard vent valve, a heat sensitive bi-metal strip is used as the valve holder. This is mounted beneath the idle vent valve arm.
The bi-metal strip holes the vent valve on its seat (closed) at temperatures below 75*. When underhood temperatures are above 75* to 85* the bi-metal strip bends upward moving the vent valve off its seat. This lets fuel vapors, caused during hot engine operation, escape from the float chamber. This results in improved hot engine idle and hot starting. At temperatures below 75*, the vent valve remains closed and retains fuel vapors internally to supply extra fuel for good cold engine starting.
During hot engine operation, when the thermostat vent valve is open, it is necessary to close the valve except at idle to maintain an internally balanced carburetor. This is accomplished through the spring steel vent valve arm which operates off the wire lever on the end of the pump lever. As the throttle valves are opened from the idle position, the vent arm exerts pressure on the bi-metal strip and forces the valve closed.
The thermostatic vent valve is adjustable to make sure it closes at the proper time during throttle valve opening from the idle position.