In carburetted engines, a float bowl in the carburettor holds fuel prior to it being fed into the engine. Fuel is pumped from the fuel tank into the bowl of the carburettor; the amount of fuel held is closely controlled by the "float". The float is a simple buoyancy device that rises or falls as the fuel increases or decreases. It controls the amount of fuel allowed into the carburettor by opening or closing a valve, depending on its float height. Adjusting floats must be done very accurately to prevent overflow of fuel or stalling.
Determine whether your float level is too high or too low. If too high, you will most likely have fuel run off through an overflow tube. If a glass sight window is present, the fuel level will be toward the upper side of the window. If your float level is too low, the vehicle will run out of fuel when driving at high speeds, or the window will show a fuel level toward the bottom of it. The fuel level should always be in the middle of the window and should not ever overflow.
Remove the carburettor or the float bowl. On motorcycles, the float bowl can often be removed while leaving the carburettor on the bike. On cars it depends on the model and location of the carburettor. Be careful not to drop or lose any small parts; the bowl and carburettor must be replaced exactly as removed.
Locate the hinge of the float and find the float needle, which will be a small metal or plastic rod that is pushed up by the bracket that holds the float. When the float is pushed up by the fuel, the float needle is pushed up, which closes off the fuel supply from the fuel tank. When the float goes down due to a lack of fuel, the needle drops, allowing more fuel in.
Push the tab that holds and pushes the float needle up 1/32 inch with a flat head screwdriver, if you have determined your carburettor float is too low. Pull it down the same distance, if you have determined your float is too high. Be very careful not to bend any other elements on the float assembly when doing this.
Replace the bowl or the carburettor, and replace the fuel line. Check the height of fuel through the window, or take a test drive and check the results. Readjust as necessary.
Carburettor manuals will dictate the height the float should be adjusted to; reference this material if available and adjust to factory recommended settings using your ruler, measuring the distance the float needle hinge is from the needle at recommended float height.
Wear eye protection to protect yourself from gasoline while working on carburettors.