The carburetor float system controls the fuel level in the carburetor bowl. Fuel level is very important, because it must be maintained to give proper carburetor metering throughout all ranges of engine operation. If the fuel level is too high in the float bowl, problems such as rich mixtures, fuel spillage from the main discharge nozzles on turns, and engine loading can result. If the fuel level is lower than the specified setting it can cause lean mixtures, hesitation on acceleration, engine surge plus engine cut-out during heavy fuel demands.
The float system operates as follows:
Fuel from the engine fuel pump is forced through the fuel inlet screen or filter, and on through the float needle seat, past the float needle, into the float bowl. Fuel flow continues until the fuel raises the float pontoon to a position where it forces the float needle against the float needle seat and shuts off fuel flow.
As fuel is used from the carburetor bowl during engine operation, the float again drops downward moving the float needle off its seat, allowing more fuel to flow into the float bowl, thereby, keeping the fuel level consistent.
A float drop tang located at the rear of the float arm prevents the float from moving too far downward. The maximum float drop must be maintained so that the float assembly will drop sufficiently to allow maximum fuel flow under heavy engine fuel demands. Some models use a float needle pull clip which is attached to the float arm and connected to the float needle valve. The purpose of the pull clip is to pull the float needle off its seat, should it stick closed due to dirt or gum formations.
There are several methods of float bowl venting used in the Model G two bore carburetor. The method and type of venting used for a particular engine will be varied in design to meet the flow characteristics desired for the engine on which it is used.
Some applications use a “fixed” external vent located on the top of the carburetor air horn just above the float bowl chamber. This vents to the outside any fuel vapors which may form in the fuel bowl due to engine heat during periods of hot engine operation. This keeps the vapors from being forced into the carburetor bores thereby causing poor idle and hard starting due to excessive richness.
An internal vent located inside the air horn bore just beneath the air cleaner, is used to help balance the pressure from beneath the air cleaner to the fuel in the float bowl. The amount of fuel metered by the carburetor is dependent upon the pressure in the float bowl causing fuel to flow. By locating vents internally below the air cleaner, the carburetor automatically compensates for built-in air cleaner restriction hence a balance can be maintained between the air flowing thorugh the carburetor venturi and the air acting upon the fuel in the float bowl. If the carburetor is also equipped with a fixed external vent, it will not be completely balanced but the pressure difference will be less, so consequently, the internal vent tube will very must effect carburetor calibration. Fuel vapors will also be drawn from the area above the fuel in the float bowl by the internal vent during engine operation and is compensated for in the calibration of the carburetor unit.
The float system on some models has an atmospheric idle vent valve which is normally open only during the engine idling period and when it is shut down. THe idle vent valve is used in place of the fixed external bowl vent and is operated by a tang on the pump lever. When the throttle lever is in the idle speed position the idle vent valve is open, this allows any fuel vapors in the float bowl during periods of hot engine idle and hot soak to be vented to the outside. The vent valve closes when the throttle valves are opened to the off-idle position at which point the carburetor returns to an internal balance by the internal vent tube which transmits air pressure from beneath the air cleaner to the fuel in the float bowl.
It is necessary that the idle vent valve be closed during all periods of operation except at idle, otherwise excessive richness can be caused by the higher atmospheric pressure acting upon the fuel in the float bowl.
Some units are equipped with an interal fuel inlet filter located behind the fuel inlet fitting. On these units the filter element is spring loaded. This feature provides a pressure relief so that in the event the filter should become clogged, the restriction will cause fuel pump pressure to overcome the filter relief spring and allow fuel to by-pass the filter.
Most of the 2 jet kits will include a clip that attaches the float needle to the float. This is used to facilitate lifting the needle out of the seat allowing fuel to enter. Helpful when the needle wants to stick. Watch closely how you install this clip so when done, the needle is pulled straight up and down and not pulled to the side.
Rochester 2 Jet Float Adjustment (includes Mercarb)
Float Level Adjustment
Float Drop Adjustment
With float hanging freely, measure from gasket surface to lip of plastic float, or bottom of metal float.
Check the float drop to make sure the needle doesn't fall out, or gets stuck to the side.