In some ways, a Model A carburetor’s fuel bowl is like a toilet. If a toilet tank’s float is not set low enough, it allows too much incoming water and may even allow a situation where water escapes through the overflow pipe.
Similarly, a high fuel level can occur when the carburetor float is set too high. It eventually shuts off the valve, but allows fuel to spill out of the cap and main jets.
A too low fuel level can starve the engine of gasoline because fuel from the cap and main jets must be drawn up by the acceleration of air through the venturi that lifts fuel up while mixing it with the air in a process called atomization. Some people call this suction, I call it vacuum.
The float must be set just right. This is done by adding or subtracting gaskets where the gas shut-off needle valve screws into the bottom of the upper casting near the fuel line entry.
The trick is to set the float function close enough to allow sufficient fuel to enter the bowl freely but not so much as to “push” fuel to where it will rise and spill out of the tips of the cap and main jets. The float also has to positively stop all fuel flow to avoid over-spilling at the cap and main jets.
Do not under any circumstances bend or tamper with the float design or bend the tab that actuates the valve. This will negatively affect float function and could break a solder joint or create a leak into the float itself. It could also affect the how far the float body descends into the bowl, causing the fuel level to creep higher. A partially submerged float is a bad thing. Just adjust with gaskets, okay?
