Expert Advice
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Regulating Tillotson carburettors
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On the track, a great deal of attention is given to the kart’s engine and balance, while the carburettor is too often neglected. But the latter actually has an important influence on engine performance and therefore lap time, as well as on reliability.
With Andrew Fallon, Product Engineer at Tillotson, we reveal some interesting tricks for the correct regulation of the two carburettors for the OK and OKJ categories: the HC-112A and the HW-43ACNC.
It certainly can. In the OK category, for example, drivers are more experienced than in OK-Junior and use their brake and accelerator pedals differently. For this reason, at Tillotson we’ve designed a carburettor that’s more sensitive to external factors like driving style.
What’s really new in the OK model is the presence of the “enrichment circuit,” which keeps the engine better prepared for sudden openings of the butterfly valve. If the driver uses the air shield too much (as less-experienced drivers often do), the blend will be too fuel-rich and the engine’s responsiveness will suffer. Vice versa, a dry opening of the valve gets an immediate response from the engine, improving performance.
Actually it is. We advise against opening and closing the valve even when the engine is turned off, because opening it allows fuel to enter the enrichment circuit, which leads to an overly fuel-rich blend and a loss of responsiveness when exiting a curve.
Another trick is to avoid undue tension on the accelerator cable, which many drivers want taught for a greater responsiveness on the butterfly valve when they step on the accelerator pedal.
But by doing so, you actually risk opening the butterfly valve ever so slightly even when your foot is only resting on the pedal, and consequently raising the transparent plastic break-off plug (resistant to corrosive agents like fuel) and losing the sealing effect on the enrichment circuit, and once again obtaining an overly fuel-rich blend.
The main difference is that the OK needs a lot of fuel, while the OKJ loses responsiveness with an overly fuel-rich blend. And engine response is a particularly important aspect for drivers. So, on the OKJ carburettor it’s better to opt for a harder carburettor spring, and adjust the low-end needle to a more open position. This way you get a leaner blend at the opening of the butterfly valve, with good engine response, and an enrichment of the blend at the middle speed range, because the low-end needle (which on the carburettor for OKJ acts for 90% of the power curve) will have been set on richer blend values. For the OK carburettor, on the other hand, you need to focus on the high-end needle to help at the high speed range, or else the engine will detonate. You can’t open the low-end needle too much here, without risking engine delay in the acceleration phase. It should be said, however, that the OK requires more fuel at the mid and high-speed ranges, so the more you open the low-end needle, within certain limits, the better supply will be at the middle speed range.
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