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Rotax Carburettor Explained

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The pressure at “1” is higher than at “2” because the fl uid speed at “1” is lower than at “2”
Source: Public domain from Wikipedia (http://en>wikipedia.or g/wiki/File: Venturifi xed2.PNG)

I thought it would be a good idea to talk about how float bowl carbs like the Dellorto actually work. I know there are more than a few people out there who know that the carb mixes the fuel and air to be burned by the engine but not much beyond that.

The Dellorto Rotax carburettor works on the basic principle that air passing through a venturi (a narrowing section like the inside of a cone) causes a vacuum. The air being drawn in by the engine is sucked through the venturi in the carb, which is located where the slide is. As the slide opens, the vacuum increases. Imagine putting a drinking straw in a glass of orange juice and sucking on it. The vacuum in your mouth is drawing the juice out of the glass. The carb is designed to make use of this principle so that the amount of fuel coming from the float bowl can be accurately metered so that the engine runs with the correct amount of fuel for a given throttle position.

The fuel is sucked from the float bowl (glass) through 3 different circuits (straws) and comes out in the venturi (mouth). These circuits are known as the idle circuit, the progression circuit and the main circuit. As their names suggest these circuits control the amount of fuel at different positions of the throttle.

The idle circuit allows the engine to idle. A small amount of air is drawn into it through one of the small holes located up stream of the slide in the bell mouth of the carb. This air is mixed in the idle circuit with fuel being drawn up from the float bowl through the idle jet and idle jet emulsion tube, which unlike the main jet cannot be changed for different ones. The emulsified fuel mixture is then sucked through the furthest tiny hole from the slide.

As the slide opens, the responsibility for feeding the engine with the correct air/fuel mixture is taken over by the progression circuit. As with the idle circuit some air is drawn in from the small hole in the bell mouth and the fuel is drawn through the idle jet but this time the emulsified air/fuel mixture emerges from the small hole that is just behind the bigger hole where the needle goes in the main circuit.

At this point the slide is only open about one quarter. As it continues to open all the way to full throttle the main circuit takes over the metering of the fuel. The volume of fuel passing through the main circuit is controlled by the main jet and by the tapered needle. The needle is necessary to control the volume of fuel at part throttle opening because otherwise there would be too much fuel coming up from the float bowl if it was just left to the main jet to control it.

According to the rules, the size of main jet and the position of the tapered needle are all you can adjust. A general rule of thumb is that bigger jet sizes are used when the air temperature is cold and the air is dense. Smaller jet sizes are used when the air is warmer and less dense. Humidity plays a part in jetting too and generally one can jet down if humidity is high and jet up if humidity is low.

The tapered needle is used to fine tune the jetting in the low mid to high rpm range once you are happy with your main jet. Its always worth taking the needle from its lowest (lean) position to its highest (rich) position while on the same jet just to see how the engine responds.

It is rarely worth taking into account what jet the other drivers are running in their carbs because the fact is that there are too many other variables affecting how jetting influences engine performance, including the drivers driving style and their weight. Driving style would include how the driver controls the throttle. As fuel metering transitions from the idle to the progression to the main circuits it is best to try and get a feel for what the best rate is to open the throttle at to give the carb the best chance of work. This takes practice as does finding what jetting works for you. Having a strategy when jetting at the circuit is the best way to get to know your carb and its characteristics.