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TKART magazine Tech Talk | The diaphragm carburetor
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Gianluca Covini
25 January 2023 • 11 min. read
More compact and more versatile than the bowl carburetor, it allows the carburetion to be adjusted directly while driving. In this article we analyse its components and how it works, without missing out on some… Curiosities!
The diaphragm carburetor is a type of compact carburetor, used in almost all FIA Karting competitions for direct drive engines (such as OK and OKJ as well as the most recent national classes, OK-N and OKJ-N), but also in various one-brand championships such as the IAME Series, the ROK Cup or the Easykart Trophy. It has construction features that make it very versatile, above all because it can be adjusted directly by the driver on the track (while driving), using the adjustment screws located on the side of the main body. Inside, the diaphragm carburetor integrates a fuel strainer, a diaphragm pump and a second diaphragm (called the main diaphragm) which acts directly on the fuel dosage.
Furthermore, the main air passage duct, which arrives from the intake silencer and goes towards the engine, has a first concave and then convex shape, thus giving rise to a Venturi duct. Precisely, this is due to the phenomenon of depression which is the basis of the functioning of the carburetor. Inside the duct, after the narrowing, a throttle valve is positioned, controlled by the accelerator pedal. As we will see later in detail, the main diaphragm, deforming due to the depression in the air duct, acts on a spring lever mechanism which allows the opening of a needle valve, which, in turn, opens the passage to the fuel which fills a small chamber called the "dosing" chamber. This chamber has supply holes, which allow the fuel to reach the air duct and mix with it before reaching the combustion chamber.
1. Closing screw of the fuel strainer; 2. Fuel strainer cap; 3. Fuel strainer gasket; 4. Fuel strainer; 5. Fuel pump closing screw; 6. Fuel pump body; 7. Fuel pump diaphragm; 8. Fuel pump gasket; 9. Main diaphragm cover; 10. Main diaphragm; 11. Main diaphragm seal; 12. Fuel metering lever mechanism; 13. Needle valve and its seat; 14. Low-speed (left) and high-speed (right) jets; 15. Low speed (left) and high speed (right) adjusting screws; 16. Carburetor body; 17. Gas regulating butterfly valve; 18. Throttle tension springs (primary and secondary); 19. Throttle cable lever; 20. Throttle rotation shaft (with clip and retaining screw); 21. Throttle lever screw.
The diaphragm carburetor can theoretically be mounted at any angle, even upside down, because it has no float inside and therefore does not work by gravity (precisely for this reason we find diaphragm carburetors also in the 2-stroke engines of chainsaws and brush cutters, even if with certainly lower performance characteristics). In fact, there is only the mixture in the small dosing chamber inside the carburetor, separated from the air by the diaphragm, which prevents the two elements (air and fuel) from emulsifying and keeps the quantity contained constant. In karts it is common practice to mount the diaphragm carburetor with the mixture supply in the upper part, so as to have the adjustment screws on the driver side. Diaphragm carburetor bodies are mainly made by die-cast aluminium alloy, sometimes alloyed with magnesium to lighten the weight.
The upper cover, where the mixture enters, is instead usually made of plastic. There are few manufacturers of this type of carburetor compared to the manufacturers of bowl carburetors, which are much more widely used because they are also used in the motorcycle industry. The reference company when it comes to diaphragm carburetors is the Irish company Tillotson (although it was founded in the United States of America, in Ohio, in 1914), but there are also other important brands such as Tryton, Walbro, Ibea and JHC extension. The size of the carburetor is identified by the minimum diameter of the duct, at the narrowing point of the Venturi channel. Commercially available diaphragm kart carburetors range from 20-22 mm (for MINI karts), up to 30 mm for OK and OK-J. But there are also 35 mm ones, for example the Tillotson HB-10A specially developed for the IAME X30 Super 175 cc.
In the picture the Tillotson HC-112A carburetor, with a 24 mm diameter Venturi duct. This is the carburetor model used on the DPK Racing team OK 2022 world champion kart. It is true that some single-speed classes (for example: Rotax and ROK Cup) use float-type carburetors instead.
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