Bing 64 (CV) Carburetor Part 1
This article will focus on the Bing 64 CV (Constant Velocity) carburetor. The basic principal of operation utilizes a vacuum operated slide that varies the venturi size which, in turn, maintains a constant velocity of air passing through the carburetor at all engine power settings. The advantage of the CV carburetor is that it supplies the engine only as much fuel/air mixture as the engine demands. For an aircraft applications, where we have large excursions in altitude, this is exactly what the doctor ordered. The Bing 64 carburetor (Figure: 1) has become, hands-down, the most popular carburetor used in the light sport industry. It is used on both the Rotax 912 as well as the 914. It is also used on the HKS 700 E, the Stratus, the Rotec Radial, and the Jabiru engines. This carburetor has a long history of great reliability, on a plethora of aircraft.
The carburetors are easy to maintain with very few moving parts, rely on good design, and relatively simple operation to garner their great reputation. Most of the problems that we see on the CV carburetors are created by the operators, inadvertently and primarily from a simple lack of knowledge on basic operation, maintenance, and troubleshooting. In our Light Sport maintenance classes, there’s one universal area that will cause a student to sit up straight and pay attention: Troubleshooting! We tell the story constantly about how easy it is to troubleshoot an engine problem. (For this article we will say engine/carburetor problem). It’s as simple as this. Step 1: Inspect the engine/carburetor to find out what has changed from the stock configuration. Step 2: Change it back to stock configuration. Step 3: Run the engine to verify that everything works perfectly. Now, we may be joking a little bit about this whole process, but it really is true. Most of the time when a customer brings airplane with an engine/carburetor to us, that isn’t working properly. We simply hunt to find out what has or hasn’t been done to the aircraft that now makes it different than when it was a new engine. Keep in mind that these engines are all manufactured on the exact same assembly line, under extremely controlled conditions. If we can simply put the engine/carburetor back to the same configuration which it was when it rolled off the assembly line, it is, once again, going to operate like a “new engine”. Of course, the problem with this whole concept is having the requisite knowledge to be able to easily identify what is no longer in the “stock configuration.” In order to make good troubleshooting decisions, we need to start with a good understanding of the CV carburetor’s theory.
There are 3 primary elements necessary for making an engine run, fuel, air, and spark. Two thirds of this equation is the responsibility of the carburetor. The job of the carburetor is to supply, not only the total quantity of fuel and air (power), but also to supply the correct ratio of air/fuel (mixture) for each power setting selected by the pilot. The pilot can select a power setting on the CV carb by moving the throttle lever which is hooked directly to the throttle assembly. However, unlike a conventional carburetor, this does not automatically allow air and fuel into the engine. You can think of it like the captain of a ship sending a signal to the engine room. He moves the levers on the bridge which in turn sends a signal to the engine room telling them to add more power. By opening the throttle valve we reposition the throttle valve, however, the piston (not controlled by the pilot) is in control of how much air flows through the carburetor. The CV carb works on the principal of varying the pressure differential above and below the piston, which, in turn, varies the venturi opening like a conventional slide carburetor. During the engine intake stroke, a vacuum is presented to the carburetor via the intake manifold. This low-pressure draws air through the carburetor. (Figure: 2) The motion of the air, under the slide (piston), acting as a venturi creates a pressure drop. This pressure drop is transmitted through passages in the piston to the enclosed chamber above the piston. The upper and lower half of the piston is separated by a rubber diaphragm that allows the movement of the piston. The pressure below the piston and diaphragm is vented to atmospheric pressure via a passage from the inlet face of the carburetor. The greater the pressure differential, the higher the piston rises, the larger the venturi opening. Fortunately, unlike the ship metaphor, the signal transmitting from the throttle valve to the piston is smooth and nearly instantaneous. It is, however, one of the reasons that the Bing carburetor can get away without the utilization of an accelerator pump. If the pilot were to “jump” on the throttle suddenly, the inertia of the piston limits how quickly it can respond. Additionally, when the venturi is suddenly exposed to the throttle being opened, the velocity of air in the venturi increases significantly, creating a lower pressure. This sudden pressure drop is not only the pressure that is transmitted to the top of the diaphragm that causes the slide to move up, but it also creates an increase in vacuum. This vacuum at the diffuser draws additional fuel from the needle jet providing for a richer than normal mixture that is needed for the acceleration process. In transitioning to talking about fuel, we need to be aware that the fuel distribution is also out of the hands of the pilot. Unlike a conventional aircraft, the Bing carburetor has no mixture control for the pilot to manipulate. The fuel distribution from idle to full throttle is also automatically controlled within the carburetor. Except for the Idle and choke circuit, the fuel is injected into the carburetor body at the diffuser. This component is also known as an atomizer. The purpose of the diffuser is to inject atmospheric air from the lower inlet of the carburetor at the exact location that fuel is exiting the needle jet. This breaks up the fuel into very small (atomized) particles, which enhances the distribution of fuel within the intake manifold, as well as improves the combustion process.
The distribution of fuel is regulated by several systems. The idle jet, the needle jet and jet needle working together, and the main jet have control over the mixture at various segments of throttle setting. There is overlap within the systems from idle to mid range and from mid range to full throttle. (Figure: 3) Once we are above the 25% throttle setting, all of the fuel is routed in series through an assembly of fuel distribution components. Starting at the bottom of the float bowl, the fuel travels upward through each of the components starting with the main jet. The main jet is screwed into the mixing tube. The mixing tube is screwed into the lower portion of the carburetor body holding the needle jet into the diffuser which, in turn, is pressed into the carburetor body. Simultaneously, the jet needle is being automatically positioned by the piston within the needle jet. As the piston rises, so dose the jet needle. This provides for an ever increasing amount of fuel that corresponds with the amount of air that is being allowed into the engine. When the piston and needle rise high enough, the orifice size from the needle jet / jet needle becomes larger than the orifice size of the main jet. Because these components are in series, the smaller of the orifices become the controlling factor. The resistance to fluid flow in each of the jets contributes to the overlap from mid range to full throttle.
A typical slide carburetor, where the pilot controls the piston via a direct cable connection to open and close the throttle, is at a great disadvantage. As the aircraft gains altitude, the air density diminishes whereas the fuel density remains the same. This inevitably results in a rich mixture. The CV Carburetor on the other hand, senses the reduced atmospheric pressure on the lower half of the diaphragm, and the piston lowers in the body of the carburetor. This happens automatically even though the throttle valve position is still in the full open position. Because the piston is lowering the needle jet / jet needle restriction also automatically reduce the fuel to lean the mixture. You can start to get a sense for the brilliance of the design. In part 2 of this article, we will dig even deeper into the inner workings of the CV carburetor. Once you have a complete understanding of the carburetor, you can’t help but have a great deal of confidence in your engine. After all, the carburetor is controlling two of the three things needed for your engine to run, fuel and air.