Rotax jetting in hot, cold and wet weather: what changes on track
Updated: 3 days ago
The same jetting that works perfectly in the morning may no longer work in the afternoon. The engine is the same: it is the air that changes. Here you will find the Rotax MAX base setting, what each carburettor adjustment does, how to choose the jet in hot, cold and wet weather and how to change it at the track.
In short The standard Rotax MAX calibration refers to 25 °C ambient temperature and 400 m altitude. With different temperatures or altitudes you change the main jet, calculated with the Rotax MAX Jetting App. The carburettor base setting is the needle clip in position 2 and the air screw at 2 and a quarter turns out from fully closed. A lower clip position (1) makes the mixture leaner, a higher one (5) richer. The main jet is changed without removing the carburettor, after draining the float chamber. In cold weather bring the water up to temperature before pushing.
Why Rotax jetting changes with the weather
The carburettor meters fuel according to the air entering the engine, and the air changes with temperature, pressure and altitude. The standard Rotax MAX calibration is made for 25 °C and 400 m above sea level. The standard main jet suits almost all conditions, but with different temperatures or altitudes the main jet must be adapted.
On the Dell'Orto VHSB 34 XS carburettor the adjustment that follows the weather is the main jet.
The carburettor adjustments and what each one does
• Main jet: it controls the mixture at full throttle and is the one you change with the weather. The size is printed on the face of the jet. To have every size there is the Dell'Orto main jet kit.
• Needle: the standard clip position is 2. In position 1 the part and full load mixture becomes slightly leaner, in position 5 slightly richer. The needle is the Dell'Orto K57 needle and the position is changed by moving the Dell'Orto needle clip.
• Idle speed screw: screwing it in raises the idle, screwing it out lowers it.
• Air screw: it sets the idle mixture. Screwing it in makes the idle richer, screwing it out leaner. The base setting is 2 and a quarter turns out from fully closed.
Hot, cold and rain: what to do on track
• In hot weather: between morning and afternoon the temperature can change a lot. Redo the calculation with the current data and check whether the recommended jet changes, instead of keeping the morning one.
• In cold weather: below 10 °C ambient temperature do not demand full power until the water has reached at least 45 °C. For the best performance the water must reach 55 °C: if it does not, partly cover the radiator, as explained in the guide to the radiator blind and deflector. For driving in the cold read how to drive a kart in winter.
• In the rain: when the rain arrives temperature and pressure change: update the data and recalculate the jet before the heat. For lines and tyres in the wet read how to drive a kart in the wet.
In all three cases the rule is the same: do not guess, recalculate with the data of that moment.
How to use the Rotax MAX Jetting App
• Choose the engine: from the Rotax 125 Micro MAX up to the DD2.
• Automatic: the app uses GPS and internet to get the position and weather data from the nearest station. iOS and Android use different stations, so the data may differ slightly.
• Manual: enter temperature, altitude, atmospheric pressure at sea level and humidity. If a value is not realistic the app shows it in red. If you use your own barometer, it must be calibrated to the altitude you are at.
• The other settings: the INFO button in the app covers float height, needle position and air screw setting.
• Fuel: the calculation is based on 95 octane fuel. With higher octane fuel a different main jet from the recommended one may be needed.
• Responsibility: the app does not release you from the responsibility of choosing the right jet, and Rotax does not grant warranty if wrong jetting damages the engine.
How to change the main jet, step by step
• 1. Tools: a 19 mm open-end wrench and a flathead screwdriver. The carburettor stays on the engine.
• 2. Drain the float chamber: remove the drain screw at the bottom of the float chamber with its gasket. The drained fuel can go back into the tank.
• 3. Remove the jet: unscrew the main jet and its Dell'Orto main jet washer, part 261550.
• 4. Fit the new jet: refit the washer and the jet calculated with the app, then close the drain screw with its gasket, hand tight.
• 5. Restart: after a cold start the fuel pump takes a few seconds to fill the float chamber.
The main jet: what the regulation says
In the Rotax MAX Challenge any Dell'Orto main jet is legal, even if it is not in the Rotax catalogue. Organisers can set a minimum main jet for each race, and jets must always be properly tightened. The other carburettor parts are mandatory instead: you will find them in the article on the Rotax engine seal and technical regulations. The complete carburettor guide is the Dell'Orto VHSB 34 XS guide.
Measuring the air directly at the track
To get the exact data of the place where you race there is the AirPro air density meter: it measures temperature, pressure, relative humidity, relative altitude and air density, and stores the session data. These are the same data to enter manually in the Jetting App.
If the engine does not run as it should even with the right jet, read kart engine bogging at corner exit or message us on WhatsApp.
Frequently asked questions
How does Rotax jetting change with the weather?
The standard Rotax MAX calibration is made for 25 °C and 400 m altitude. With different temperature, pressure or altitude you change the main jet, calculated with the Rotax MAX Jetting App.
What is the base setting of the Rotax carburettor?
Needle clip in position 2 and air screw at 2 and a quarter turns out from fully closed. The main jet is chosen according to the conditions with the Rotax MAX Jetting App.
How do I change the Rotax main jet?
Without removing the carburettor: drain the float chamber by removing the drain screw, unscrew jet and washer with a 19 mm wrench, fit the new jet and close the drain screw hand tight.
Which main jet is allowed on the Rotax?
Any Dell'Orto main jet is allowed, even if it is not in the Rotax catalogue. Organisers can set a minimum main jet for each race.
Source: Rotax 125 MAX evo operator's manual (2024 edition), Rotax MAX Jetting App and Rotax MAX Challenge 2026 global technical regulations with Bulletin 04.
Questions about this topic?
Message us on WhatsApp: we answer personally, with the experience of people who live at the track. And if you want to drive a real racing kart, with Arrive & Drive you get into one of ours, ready on track. The right spare part? You will find it in the shop.
Arrive & Drive Pro by TKS RaceLab: 2026 karts with Rotax Senior MAX and DD2 engines, Alfano 7 telemetry and trackside technical support at Lonato, Franciacorta, Cremona and Ala. Message us on WhatsApp for information and availability.
TKS RaceLab | Performance Engineering Karting | tksracelab.com
Read also: Dell'Orto VHSB 34 XS: complete setup → · Rotax engine seal and technical regulations → · Kart engine bogging at corner exit → · Rotax Plasma cylinder 2027: what changes →
📞 Contact
📞 +39 352 0280 133
Author: Sergio, TKS RaceLab | Published: 18 October 2026
TKS RaceLab is the Italian online store dedicated to karting: spare parts, accessories and everything you need to get on track. With Arrive & Drive you arrive at South Garda Karting (Lonato), Franciacorta, Cremona or Ala and find your kart ready to go. No hassle, just driving.
Comments