Carb101: Diaphragm Carburettors

We saw in an earlier post that the diaphragm carburettor overcame some of the issues concerning the earlier carbs that used float chambers. Let's take a closer look at these carbs after the jump...

The diaphragm carb actually uses two diaphragms - an impulse diaphragm and a metering diaphragm. The role of the impulse diaphragm is to pump the fuel from the tank in to the metering chamber, with the impulses being provided by a line running from the crackcase to the carburettor; as the piston moves up and down the cylinder, changes in the air pressure within the crankcase are transferred to the carburettor. These pulses of air essentially cause the diaphragm to pump up and down, thereby pumping the fuel.

As this diaphragm is driven by pressure pulses from the crankcase, the pump will work at any angle.

A second diaphragm resides in the metering chamber which could be considered a reserve of fuel, waiting to be mixed with the incoming air dragged in via the air filter. The photo to the left shows the metering chamber, with the rubber diaphragm removed (but shown next to the carburettor).

That air is pulled in by the action of the piston moving down the cylinder. The carburettor is constructed so that the air flow speeds up as it enters the carburettor via the venturi (it's akin to squeezing a hose as water travels through it - restricting the flow speeds the water flow). The really clever bit is that increase in air flow actually creates a low pressure that drags the fuel in to the air flow, and hence is passed in to the engine. The more the throttle is opened, more air flows in, more fuel is combined and the engine speeds up. Closing the throttle shuts off the air flow, less fuel is drawn in and the engine slows down.

There is a problem with this though - at idle speeds the air speed is not fast enough to create enough of a low pressure to pull the fuel out, so a slightly different approach is taken. For now though, that's it - we'll take an even closer look at exactly how it all hangs together in the next post.