Rotax jetting theory

Dellorto carburetor

Rotax MAX Dellorto Carburetor

The carburetor is one of the most important parts in a karting engine.

It's main function is to form a "proper" homogeneous inflammable mixture of fuel and air. This fuel-air mixture is formed through vapourising and by uniformly spraying fuel into the airstream or at least by atomising it into very small droplets.

In order to get this "proper" mixture of gas and air, several adjustments has to be taken into consideration, as venturi insert size, floats weight, floats height, needle, jets (main, idlle, chocke), etc.

venturi effect

For the Rotax Max engines family some of this adjustments are fixed by the MAX Challenge regulations, as the venturi size, floats height, idle or chocke jets, etc. So in order to get the "proper" air/fuel mixture, only a few changes can be made.

In a Rotax engine, this is the standard configuration


Carburetor: Dellorto VHSB 34QD or QS

Venturi insert: 8.5 or 12.5 (older)

Needle tube: FN266

Needle: K98 or K27 (older)

Floats weight: 3.6g or 5.2 (older)

Low speed jet size: 60 or 30 (older)

Insert: B60 or B30 (older)

Needle and seat: 150 (on DD2 only: a 200 needle and seat is optional)

Main jet: free

Needle position: free


The following two combinations of inserts, low speed jets and floats are allowed:

★ Combination 1 using 12.5 insert (old combination):

Floats marked 5.2 gram with low speed jet 30 and insert B30

★ Combination 2 using 8.5 insert (new engines):

Floats marked 3.6 gram with low speed jet 60 and insert B60


You can find more technical information here. Carburetor info


With this rules, the principal adjustments to work are main jet and needle pin in order to get a proper homogeneous inflammable mixture of fuel and air at different throttle positions.

We have spoken of a "proper" mixture because the mixture strength is defined as the amount of air mixed with a fuel unit, must have a precise value,ie it must be within the limits of inflammability so that the mixture can be easily ignited by the spark in the combustion chamber. This proper value is also called stoiciometric mixture ratio.

Thought the jet size. is easy to calculate the amount of fuel in the mix, but it's also necessary to calculate the amount (mass) of oxygen. This amount of oxygen is calculated through the density of air, that depends of the current pressure, humidity and temperature.

KartMAX results

KartMAX calculate the appropriate (theoretical) jet size given temperature, humidity and pressure. Some weather stations or weather services over internet don't show "current pressure" but "sea level pressure", if it's the case, it's necessary to know current altitude to transform sea level pressure to the current one.

"Theoretical" jet value

Carb fluid

We said "theoretical" jet value, because this jet, in practice, is not accurate for all Rotax Max engines. Rotax engines and Dellorto carburetors are mass produced, they aren't two equals, regular gas doesn't have the same octanes in all gas-stations in al countries, different oils, etc. This is why it's very important to know how your engine performs against the theoretical value proposed by KartMAX. A slider is provided in the App in order to easily adjust KartMAX with your engine settings.

The only way to get the right value of the slider is to test it at the track. The procedure is quite simple. Configure KartMAX with the current weather and put this configuration in your carburetor. When the engine is in the right temperature, test it at full throttle. If the engine reach max rev limit, but without any detonation, this is the right value. If the engine didn't get 3/4 revs (over 11.000 in MAX) mix is too rich. If the engine gets full revs, but with some detonations or with some intermitent lacks of power, it's too lean.

After the first test, check the color of the spark, it's a perfect indicator of the mix. If it's oiled and black, the mix is too rich. If it's dry and almost white, it's too lean. Perfect color is almost dry and a light brown color.

Spark colors

It's also very important to use the correct spark for each temperature. It's impossible to get a perfect performance of the engine if you use a cooler or hotter spark.

Once you know if mixture is too rich or lean, adjust the slider to get a jet size bigger or smaller and test the config again. Repeat the process to get the perfect point for your engine.


This procedure only has to be made once. When you know the position of the slider for your engine it's not necessary to test it again.

Needle position

K98 needle

One common mistake with Dellorto carburetors, is to made the assumption that the needle position has a linear relationship with the main jet, and use the height of the needle as a intermediate size of the main jet.

It's also important to remembered that carburetor jetting is determined by the throttle position, not engine speed. If the engine is having troubles at low rpm (idle to 1/4 throttle), the air screw is the likely problem. If the engine has problems between 1/4 and 3/4 throttle, the needle is likely the problem. If the engine is running poorly at 3/4 to full throttle, the main jet is the likely problem.