 A carburetor is one of the most refined components of a dirt bike. Years and years of development in the racing world have made this component a complete work of art. But how exactly do they work? And how different is a flat CR carb from a mechanical slide carb? Stay tuned, find out! A carburetor has a simple job to add fuel to the air that goes into the engine. The air fuel mixture is burned in the combustion chamber and the rapid expansion of the extremely hot gases is what provides the drive of the engine. However, there's a magical number for an engine to work the most efficiently possible. An air-fuel ratio of 14.7 to 1. But what does that mean? For every 1 gram of fuel there has to be 14.7 grams of air, since air is not just made of oxygen. Any air-fuel ratio under 14.7 is considered a rich mixture and anything more than 14.7 is considered a lean mixture. However, in dirt bikes the ideal air-fuel ratio is closer to 12 to 1, since dirt bikes are performance oriented machines rather than the epitome of efficiency. But how exactly does a carburetor mixes air and fuel? Let's first take a look at the simple mechanical slide carb. A carburetor with a mechanical slide is found on two strobe dirt bikes and the way it works couldn't be simpler. As the name implies, you have a mechanical slide crossing the main air passage, which is attached to the throttle cable. How much you twist the throttle determines how much the slide is blocking the airflow in the main air passage. The section right under the slide is called the throat. Under the main air passage there's a float pole, which is connected to the fuel tank. This float pole is responsible for keeping available at all times a steady level of fuel under the throat in order to prevent irregular throttle response. The float is a simple buoy with a needle that blocks the fuel tank's feeding port when there's enough fuel in the float pole. From the float pole the fuel can be sucked into the main air passage through three different feeding circuits. The choke circuit, the idle circuit and the main circuit. Let's see how all these circuits work together. To reach in the mixture on startup, we pull the choke, which provides additional fuel to make it easier for the engine to start when it's cold. Once the engine starts, the choke closes and the idle circuit kicks in. The circuit provides a small but steady amount of fuel whenever the engine is running on idle and a little over idle through the pilot jet. When you twist more than around 1 eighth of throttle, the main circuit steps in, which provides the main amount of fuel according to the throttle response through the main jet. The principle is simple. When the slide lifts, the engine sucks air through the throat. Since the air is flowing from a larger cross section to a smaller cross section, the air has to accelerate, which in turn reduces the pressure where the cross section is the smallest. This low pressure point is placed right on top of the main jet, which creates a vacuum and sucks fuel through the circuit. To provide some adjustability, there's the adjustment screw in the idle circuit. You can either find an air adjustment screw or a fuel adjustment screw. Both make the idle mixture leaner or richer depending on the running conditions, but are adjusted in reverse. Quite simple, right? Besides the mechanical slide carb, there's also the flat CR carburetor, usually found in 4-stroke dirtpikes. This carb isn't as simple as the mechanical slide one, but it was a game changer for the development of 4-stroke dirtpikes. The flat CR carburetor also has a float bowl, an idle and main jet, adjustment screw and a slide. However, this slide has 4 small wheels and a vacuum release plate. So, what is the purpose of making the slide so complex? As opposed to 2-stroke engines, 4-stroke engines create much bigger intake vacuum pulses. If a mechanical slide is used on a 4-stroke engine, the slide would get pushed so hard against a carburetor that it would get stuck and wouldn't move freely. However, by having a slide with a fixed cap between a carb and a slide, there would be an unwanted air bypass between these 2 components and there wouldn't be any way to get around it. This is where the vacuum release plate gets the job done. When the plate is in its place, there is only a minor play between the slide and a carb, mostly thanks to the rubber seal on the inside that acts like an air valve. Near the bottom of the plate, there's a hole. This plate works as a buffer to regulate the resisting force between the slide and the carb during the vertical movement. During air intakes, the plate is pulled against the carb and sucks all the air from the space between the plate and the slide through the hole. As this happens, the rubber seal doesn't let any air in from the sides, creating a vacuum as strong as the vacuum created by the intake, pulling the vacuum release plate to the slide. This counterbalances the contact forces and allows a smoother travel of the slide while accelerating and decelerating. Incredibly simple, right? Even with this clever system, the suction created is so strong that other systems had to be developed to further improve throttle response. The double cable throttle system on four strokes is an extra way to help the slide move more easily on a carb. While one cable is pulling, the other is pushing. However, these features only address the mechanical part of the problem of having a carb in four stroke dirtbikes. On sudden and quick accelerations, there can be a lag from the moment you open the throttle to the moment the fuel is being sucked by the engine. The mixture will momentarily become too lean and can make the engine stall in typical four stroke fashion. This is when the flat CR carb brings another trick to the table, the accelerator pump. The accelerator pump is basically a rod with a diaphragm and a spring at the end, filled with fuel. It is connected to two canals, a longer but less restricted canal which is connected to a small brass nozzle on the main air passage and another shorter and more restricted canal connected to the float pole. On normal accelerations, the fuel pushed by the diaphragm will easily flow to the float pole without reaching the brass nozzle. However, on really quick accelerations, the shorter canal will be too restricted to let all the fuel reach the float pole and will be forced to go up the longer and less restricted canal. This will squirt additional fuel into the inlet manifold, preventing the air-fuel ratio from becoming too lean. This mechanism eliminates the so-called bogging, making it more suited for racing applications. A carburetor is a very simple and well engineered component and even with fuel injection becoming a standard, sometimes you simply can't beat the simplicity, performance and easy to nobility of a carb. I hope it cleared the fumes around this topic and if you want to know how jetting works, click on this video. Thank you for watching and don't forget to subscribe.