2. The Carburettor Dismounted

This is the front of the dismounted carb. Visible is the oval-ish choke paddle. The choke lever, here on the right of the picture, moves this paddle over the main airway. Restricting the air in this way makes an artificially rich mixture which can help start the engine in certain circumstances.
I'm not sure which circumstances they are though, as the use of any form of choke on any engine I've ever used has invariably made me turn up late and stinking of petrol, regardless of the weather or the time of year.

The circular airway is the chamber in which the fuel and air are mixed. The air is sucked in as the piston in the combustion chamber rises, compressing the previous charge before igniting it.

The large round slider has inside it the needle valve, which controls the amount of petrol released to the mixture.

This is the left hand side of the dismounted carb. You can clearly see the throttle arm on the far side and the choke arm on this side.

The lower brass fitting is the fuel connector, which screws in to the carb body. The fuel feeds directly into the float bowl, which is the rounded, lower half of the carburettor.

The brass fittings visible above the body are part of the throttle assembly. The throttle arm raises and lowers the slider that surrounds the needle valve.



This is the right hand side of the carb. Visible is the throttle arm, here almost fully open. The screw that is visible in the middle is what I assume to be the method for establishing the idle speed.


As you can see here, when that screw is fully wound in, it protrudes into the airway and stops the slider. A groove cut in the side of the slider works with the rounded profile of the end of the screw to provide gradual adjustment.

This provides a lower position of below which the needle valve may not go, for a certain amount of fuel to be available for the fuel-air mix, even when the throttle arm is fully down. Wind the screw out and the slider, and so the needle valve, can be lowered thereby reducing the fuel in the mix and so the engine speed.