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TKART magazine Under Review | Tillotson HW-43ACNC and HC-112A carburettors
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10 October 2017
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CIK-FIA 2016 – 2024

Federation approval is also required for carburettors. Approval for the Tillotson HC-112 and HW-43 models date back to 2016 and will be valid until 2024.


The following are the two types of carburettors used in karts: needle or membrane. OK and OKJ classes use membrane carburettors, such as the Tillotson HC-112 and HW-43


This is the typical valve of membrane carburettors (in fact, also called "butterfly" valves). There are other types of valves such as ball or guillotine valves (which pin carburettors have)


The HC-112A and HW-43ACNC are manufactured by full-machining using numerical control machines. The fusion method was used for the OKJ class, up to the previous Tillotson model.

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20 and 24 mm

DIAMETERThe maximum carburettor diameter limits for the OKJ (20 mm) and OK (24 mm) classes are established by the CIK-FIA.

Tillotson is one of the world's leading carburettor manufacturers. Its models (which, in addition to the 2T engines for karts, are also used for the ultra-light and power generator industries) have always been exclusively of the membrane type. The HC-112A and the HW-43A-CNC are the key carburettors of the Irish company for the OK and OKJ class karts.

These are two carburettors approved in 2016 and are manufactured by machining from a solid block with numerical control machines. The "CNC" logo that appears in the model name for the OKJ class indicates this type of process, used by Tillotson for the first time for this product. The advantage of machining from a solid block is that it achieves greater uniformity of the mechanical characteristics and, therefore, carburetion at different temperatures.
This is without doubt costlier than fusion, since it is carried out using sophisticated 5-axis numerical control machines that only produce 3 carburettors per day.

Aesthetically, the HC-112A and HW-43A-CNC differ mainly due to their external colours: anodized blue for the OK class model; not anodised for the OKJ class.

The Venturi has, of course, various diameters, in other words the mixing pipe: in fact, international regulations set a maximum value of 20 mm for the OKJ class and 24 mm for the OK class. The diameter of the butterfly valve is also different: 26 mm for OKJ, 28 for the OK.
The HW-43A-CNC carburettor is the first model for the OKJ that Tillotson machines from a solid block. Previously, it always used fusion processing
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The HW-43A-CNC carburettor is the first model for the OKJ that Tillotson machines from a solid block. Previously, it always used fusion processing
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The HC-112A is the carburettor for the OK class, which is anodized blue in colour
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The HC-112A is the carburettor for the OK class, which is anodized blue in colour
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In both models, the highs and lows adjustment screws are located at the bottom of the side, as well as a button for thinning the carburettor during the racing formation lap. Concluding a first external analysis of the two carburettors, it is worth emphasizing the finishing of the two carburettor adjustment screws, with the letters H (High - High) and L (Low - Low) engraved on them. On the HC-112A the anodization even allows the engraving of the indication of carburetion + (enrichment) and - (thinning).
The two Venturi carburettors are very similar, with small differences in shape due to the different needs of the OKJ and OK engines. In fact, the latter requires more fuel also because of the presence of a Power Valve.
For the same reason, the carburettor nozzle for the OK is larger. Furthermore, Tillotson has produced a new nozzle for the HC-112A that adds three unused through-holes to the main hole for the passage of gasoline, capable of creating a turbulence that increases and optimises the pulverization of petrol with the air.
For the OK, the highs circuit has two petrol passage holes positioned where the Venturi diameter is minimal and, due to the laws of physics, the air flow is at its maximum speed and lower pressure. There are three holes in the OKJ version: this can limit the flow of petrol (the OKJ engine in any case needs less fuel) but optimises the spraying and distribution of petrol in the pipe.

The Enrichment circuit is an important new feature for both carburettors. It is a mixture enrichment circuit that allows the passage of petrol into the Venturi duct. For the OK, the circuit flows into a hole at the height of the butterfly valve, which, when closed, also seals the hole with the help of a small corrosion-resistant plastic gasket. As soon as the valve opens, the circuit boosts the petrol, enriching the mixture. For the OKJ, the circuit flows through a closed valve nozzle. This allows keeping the lows screw more closed, thinning the carburettor and giving a more responsive engine.

For both carburettors, there is a spare parts kit which includes gaskets and membranes (partial kit), as well as a needle valve (complete kit).
There are two notches on the butterfly valves (larger for the OK, smaller for the OKJ) that allows adding air to the mixture, thinning it and improving throttle response
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The HC-112A nozzle (above) is larger than that of the HW-43A-CNC (below). It also has three additional holes that increase and optimise fuel pulverization with the air
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The Venturi holes (2 in the OK, above, 3 for the OKJ) are located where the air flow rate is at its maximum and the pressure is at a minimum, which optimises the nebulization of petrol in the air
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The two holes of the new enrichment circuit on Tillotson carburettors for the OK and OKJ classes
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