Piston Engines - Carburettors — Page 110, Lesson 134

Let's pick up with the slow running system. At low engine speeds, the volume of air passing into the engine is so small that the depression in the choke tube is insufficient to draw fuel through the main jet. So the main jet simply won't deliver fuel at idle. But above the throttle valve there exists a considerable depression, and this is utilized to affect a second source of fuel supply for slow-running conditions. Here's the layout. A slow running fuel passage with its own jet leads from the float chamber to an outlet at the lip of the throttle valve. The strong depression at that point gives the necessary pressure difference to create a fuel flow. So instead of relying on the choke tube, we tap into the strong suction right at the throttle valve lip. The size of the slow running jet is such that it will provide the rich mixture required for slow running conditions. So the jet is deliberately sized to give a rich mixture at idle. An air bleed, opening into the choke tube below the throttle valve, assists atomization. That air bleed helps break the fuel into fine droplets. Now, the purpose of the transverse passage drilled through the throttle valve is to evenly distribute the mixture over the area of the induction manifold. So that passage spreads the fuel evenly across the manifold. A small hole is drilled into the transverse passage from the choke tube side, and acts as an air bleed to draw some of the fuel through the throttle valve to mix with the air passing to the engine. As the throttle is opened, the depression at the lip of the throttle valve decreases and the depression in the choke tube increases to the point where the main jet starts to deliver fuel and the flow through the slow running system slows down. So there's a progressive handover. Carburettors must be carefully tuned in order to obtain a smooth progressive change over between the slow running and the main system to prevent 'flat spots'. A flat spot is a period of poor response to throttle opening caused by a temporary weak mixture. It normally makes itself felt as a hesitation during engine acceleration. So if the changeover isn't smooth, you get a weak mixture and a hesitation. Now, a cut-off valve is usually incorporated in the slow running passage, and is used when stopping the engine. When the cut-off is operated, the valve moves over to block the passage to the slow running delivery. The mixture being delivered to the engine becomes progressively weaker until it will not support combustion and the engine stops. This prevents any possibility of the engine continuing to run erratically due to pre-ignition, and also prevents fuel condensing in the cylinders which would tend to wash the oil from the cylinder walls, causing lack of lubrication when the engine is next started. So the cut-off serves two purposes: it stops the engine cleanly, and it protects the cylinder walls from fuel washing away the oil. The cut-off may be a separate control or it may be incorporated in the mixture control lever. So on some installations you have a dedicated control, on others it's part of the mixture lever. That's the complete slow running system.