4. The Carburettor Disassembled

The carburettor front, from below, shows the drain screw in the bottom of the float bowl. The washer is important, as a good seal is necessary to stop fuel leaking out.

The drain screw is there to drain fuel from the float bowl without needing to remover the carb from the motor, but can also be used to diagnose fuel issues by showing the progress of fuel in the process.

You can see by eye if there's fuel in the tank, and if you have a clear in-line filter you can see if there's fuel after the tap, and if you undo the drain screw a bit you can see if the fuel is making it to the carb itself.

This screw should be accessible below the front panel of the motor, though when attached you will have to turn the motor to one side to gain access as there's usually little room between it and the transom to which the engine is attached.


Two screws hold the float bowl on to the carb body, with a gasket in between to stop leakage.



With the float bowl off, you can see the float bowl on the left, the black plastic float on the right, the float spring that holds the float needle and the brass main jet.



When the fuel passes down the fuel connector, here the lower brass-ringed circular aperture below the center hole, it runs into the float bowl.
The black plastic float rises as the level of fuel increases. The fork-shaped float spring has a small channel in it to which connects the float needle, which rises into the passage under the fuel connector.
As the level of fuel in the float bowl reaches its optimum, the float needle touches the fuel connector and stops the fuel flow until the level in the bowl is reduced and the float falls, allowing the float needle to fall and more fuel to flow.

It is important that the float is air-tight and the soft taper on the float needle is in good condition, otherwise the carburettor could flood and engine performance will suffer.

The main jet, visible here as the threaded brass nut to the left of the float spring, governs the amount of fuel that can pass to the needle valve. The hole through its center is very small and must be kept clear and in good condition, not burred or distorted. Like the needle valve, it can be changed for bigger/smaller sizes as a tuning technique, but this is irrelevant for maintenance and operation of a standard engine.