were their hands cover air box? or at the carburetor?

But whichever way that was, they were adjusting the mixture

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neat, balance and practical

Most of the participants were probably controlling the fuel intake as to not overload the carburetor when we're travelling at such a slow speed..

It is also possible that the drivers were warming up the engine to the optimum temperature quickly by covering the radiator.

...Yup, most likely the case.

So lets say they are controlling the fuel intake during slow speed, what will happen if it overloads?

Consistency. "Do one thing at a time; Jules" - EL

During the formation laps the karts are running at low RPM (often below 7,000 RPM). The karts optimum performance is at high RPM (say for Rotax Senior, 13-14,000 RPM). It's possible the engine could flood and stall as it is not burning sufficient anounts of fuel or it may cause sluggish acceleration when the race commences. There are also other factors that would have an impact on the kart performance as mentioned above, such as getting the engine to optimum temperature as soon as possible.

Da Lobster

lobster guru has spoken

As mentioned there are two main reasons drivers have their hand on the side in a Rotax Max kart:
-cover the radiator with their hand to warm-up the engines faster, usually won't do much at formation-speed because the radiator isn't getting much cooling. This is likely what the driver is doing on the first lap out of the pits when they are still going at fairly fast pace.

-then when going slow on the formation lap is pinching shut the fuel line just ahead of the carb now and then so that not too much fuel goes in. The danger with too much fuel going in when put-putting around is that you will get a slow start - or worse your plug fouls and you get a really slow start or no start at all The Rotax carb has fixed jets so you can't actually adjust the fuel mixture by hand as you can in most karting diaphragm carbs.

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