All Model B carburetors use the conventional needle and seat to control fuel level in the float bowl. The the concentric float bowl design, duel pontoon floats are used to maintain a constant fuel level. This is important as fuel level directly affects the air/fuel ratio by determining the distance the fuel must rise to enter the nozzle for the idle and main metering systems. A low fuel level will produce a richer mixture and possible cause flooding. The float bowl is designed so that the fuel is centrally located around the main well so that efficient carburetor metering can be maintained under all engine operating conditions.
As shown in the above figure components of a typical float system are the fuel inlet fitting and gasket, fuel filter and gasket, pressure relief spring, needle valve and seat, and the float. It should be noted that the fuel filter design is not used on all applications. On units using the inlet filter, the fuel filter element is spring loaded. This feature provides a pressure relief so that in the event the filter should become plugged, the restriction will cause fuel pump pressure to overcome the spring and allow fuel to by-pass the filter.
As fuel is used from the float bowl, the float drops downward and allows the float needle to come off its seat. This allows fuel to flow into the float bowl from the fuel pump. The fuel flow continues until the fuel level reaches the correct height set by the float level adjustment.
At this point, the needle again seats and fuel flow ceases. While the engine is running, the float needle is continuously unseating and seating as fuel from the float bowl is used for engine operation. Three air vents are used for transmitting air pressure to the fuel in the float bowl. Two external vents, located in the air horn just beneath the air cleaner, supply atmospheric pressure to the fuel in the float bowl and are used to remove fuel vapors which might disrupt engine operation during prolonged hot engine operation.
An internal vent tube, located inside the air horn bore, extends upward from the float bowl to the base of the air cleaner, to provide a balance between external and internal air pressures acting upon the fuel in the float bowl.
On some applications, the external vents are replaced by an idle vent valve as shown in the figure below. The vent valve is located directly over the pump plunger. When the throttle valve is closed, the top of the pump plunger opens the vent valve and allows fuel vapors to vent from the float bowl during hot idle operation. The vent valve automatically closes after the throttle valve has moved from the idle position into the off-idle range.
| VEHICLE | YEAR | APPLICATION | PRIMARY FLOAT LEVEL | PRIMARY FLOAT DROP |
|---|---|---|---|---|
| Buick | 1964-65 | Special | 1 9/32 | 1 7/8 |
| Checker | 1964-65 | 6 cyl | 1 9/32 | 1 3/4 |
| Checker | 1966 | Cab S/T | 1 9/32 | 1 3/4 |
| Chevrolet | 1932-67 | All 6 cyl | 1 9/32 | 1 3/4 |
| Chevrolet | 1967 | 230, 250 | 1 9/32 | 1 3/4 |
| Chevrolet Truck | 1932-67 | 6 cyl | 1 9/32 | 1 3/4 |
| Jeep | 1966 | 6 cyl | 1 9/32 | 1 3/4 |
| Kiekhaefer Marine | 6 cyl | 1 9/32 | 1 3/4 | |
| Oldsmobile | 1964-65 | 6 cyl | 1 9/32 | 1 3/4 |
| Oldsmobile | 1966-67 | F-85 6 cyl | 1 9/32 | 1 3/4 |
| Outboard Marine | 6 cyl | 1 9/32 | 1 3/4 | |
| Pontiac | 1941-52 | 6 cyl | 1 9/32 | 1 3/4 |
| Pontiac | 1955-56 | Canada 6 cyl | 1 9/32 | 1 3/4 |
| Pontiac | 1957-65 | Canada 6 cyl | 1 9/32 | 1 3/4 |
| Pontiac | 1961-66 | Tempest 4 & 6 cyl | 1 9/32 | 1 3/4 |
| Pontiac | 1963-67 | Acadian 6 cyl | 1 9/32 | 1 3/4 |
| Pontiac | 1967 | Tempest 230 W/AIR | 1 9/32 | 1 7/8 |
| Pontiac | 1968 | Tempest & Firebird 250 | 1 9/32 | 1 7/8 |
| Rebley Marine | 1 9/32 | 1 3/4 | ||
| Studebaker | 1965-66 | 194, 230 | 1 9/32 | 1 3/4 |
| Universal Motors | 1964 | 1 9/32 | 1 3/4 |
