Honda C90 – carburettor drain, overflow & breather tube

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There are a few other bits involved with the carburettor fuel supply: the overflow, drain screw and breather

The purpose of the drain screw is self explanatory: when it is screwed in place it blocks a small hole in the base of the carburettor and when it is loosened fuel drains out of the nozzle shown in the picture below:


In the event that the float valve is not sealing properly, or the float has got stuck, then the fuel level will rise until it reaches the top of the overflow tube where it will then come out via the drain nozzle.

If the leak fills the chamber faster than it can be drained off via the overflow, say because the overflow is blocked (the hole is rather small, I think to limit the amount of fuel that might otherwise splash out as you go over bumps in the road) then fuel can flood into the venturi above. From there the fuel can find its way down the intake manifold to the cylinder, past the cylinder rings and into the engine oil beneath.

The nozzle shown below is a breather vent and is connected by a channel to the top of the float bowl:


As we saw earlier, the carburettor works because of the difference in air pressure between the bowl and the lower pressure caused by the fast moving air in the venturi – the breather vent helps maintain atmospheric pressure in the float bowl. The breather tube that is attached to this vent is generally routed to the still air in the frame cavity below the battery. Now we are experts on the Bernoulli principle we know that this is because air passing quickly over the end of the pipe could alter the pressure inside the float chamber and effect carburettion.

The next post describes how the carburettor controls the fuel and air mixture to the engine.

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