Carburettors!

Hello Peter Shaw

The SU is described as 'constant vacuum' because there are two distinct valves in each bore. At the engine end is a flap (butterfly) throttle, which limits the amount of mixture going to the engine. Towards the outside, there is an automatic (self operating) slide which is opened by the suction so that as the suck increases, so the hole and the flow increases. This also carries a tapered needle so that as the air flow increases, the fuel flow does the same in proportion. So, within limits, the amount of suck (vacuum) over the jet is constant, and as the flow of air changes, so does the flow of fuel..

In some versions of CV carb the slide works with a rubber diaphragm, not the close-fitting piston of the SU. The result is much the same.

One advantage of such carbs is that at low engine speed you can slam the pedal down as far as you like and the slide will only open as much as the engine needs. This should make for a better mannered engine and better fuel consumption.

The science of the mechanical carburettor is complex, and all sorts of extra fittings and tricks have been developed to ensure that the mixture is always (+/-) within the magic ratios of combustibility.

The original 'carburettors' were designed to produce from a very volatile version of petrol a gas as near as could be like coal gas, for use in small stationary engines where there was no town gas supply. The mixture was controlled manually with an 'extra air' valve - just as it was with gas. Of course, the engines were mainly constant speed devices (soon to be replaced by electric motors) so even a box with wicks in could be made to work.

The same system was used, with hot-tube ignition, on early Werner motorcycles, with the engine over the handlebars (where a push bike shopping basket goes) driving the front wheel by a round, twisted (sewing machine type) belt. The stability was not wonderful, and few of these machines have survived ...

Regards, Tim

Thanks Tim. That does make sense to me.

Peter G. Shaw