Bing 64 (CV) Carburetor Part 3 (Idle Circuit)
|March 2017 Sport Aviation Magazine|
In Part 2 of this article, we talked about the starting carburetor (choke) system. We can run the engine at lower RPM settings only on the choke system, but as soon as we reset the choke system to the off position, the engine is now running on the idle circuit only. We often use this as a troubleshooting exercise. If the engine runs with the choke partially on, but dies as the choke is placed in the off position, it is an indication that the idle circuit is the culprit. It is absolutely essential that the idle circuit be set up and functioning properly. We use the idle circuit on every flight, and it is a surprisingly important system within the carburetor. Aside from the practical aspects of having a properly operating idle circuit, there are many correlations with the idle circuit malfunctioning and other engine problems, ranging from increased maintenance to engine stoppage and even engine failure.
If you follow our articles on a regular basis, you already have an insight into our underling premise that all successful troubleshooting, maintenance, and operation, comes as a result of a solid foundation of the theory and physics surrounding the subject matter. With that being said, let’s dig into the theory of the Idle circuit.
With the slide (piston) completely closed, the vacuum present at the main fuel outlet is not sufficient to draw the fuel up from the main jet, through the mixing tube, needle jet, and into the diffuser and throat of the carburetor. At low power settings we need to supplement the fuel air system with an auxiliary fuel-air system consisting of an idling air jet, Idle jet, bypass, idle outlet bore, and an idle mixture screw. (Figure: 1)