Dell'Orto VHSB 34 XS: Complete Tuning for the Rotax MAX
The carburettor is one of the most delicate components of the whole engine system. Not because it is fragile, but because it governs the air-fuel mix in every phase of driving. Wrong tuning is paid for immediately: a slow response as soon as you open the throttle, a loss of power at full throttle or, in the worst cases, an engine working outside its thermal window. On Rotax engines, this component is the Dell'Orto VHSB 34 XS.
Understanding how it works and how to tune it is one of the most important steps for anyone who wants to get serious in Rotax, in any class. In this guide we cover everything: the regulated components, the three carburation zones, the restrictors in the Micro and Mini MAX classes, the Rotax Jetting App and the Prisma Electronics AirPro air density meter.
In short On the Rotax MAX EVO the carburettor is the Dell'Orto VHSB 34 XS, the only one allowed in Rotax races. The idle jet 60, K57 needle and DP 267 needle jet are fixed; you adjust the main jet, needle clip position, idle screw and air screw, starting from the Rotax Jetting App values for your class.
What is the Dell'Orto VHSB 34 XS carburettor
The VHSB 34 XS is a flat-slide carburettor from Dell'Orto, introduced with the EVO package and racing since 2015. It is fitted to all engines in the Rotax MAX EVO range: Micro MAX, Mini MAX, Junior MAX, Senior MAX and DD2, and it is the standard reference version for all classes.
The number 34 indicates the diameter of the venturi in millimetres; the XS code identifies the version used on Rotax EVO engines. Before the EVO, Rotax engines used earlier versions of the VHSB 34 (QS and QD); today in the Rotax MAX Challenge only the carburettor with the body marked VHSB 34 and the XS code is allowed.
The internal components of the VHSB 34 XS are regulated by Rotax: needle, idle jet, needle jet and float have fixed specifications and cannot be modified or replaced with non-homologated parts. Any non-original component is a technical infringement subject to scrutineering at races.
The three carburation zones
To understand how to work on the tuning, you need to know the three zones in which the VHSB 34 XS works. Each zone responds to a different component, and each is adjusted in sequence, starting from the main jet.
Zone 1: from 0 to ¼ throttle: air screw
In this phase the carburettor uses the idle circuit. The air screw of the VHSB 34 XS is located upstream of the flat slide, on the air filter side. Opening the screw (turning anticlockwise) lets more air into the idle circuit: the mix becomes leaner. Closing the screw (turning clockwise) lets in less air: the mix becomes richer. The base position indicated by Rotax is 2¼ turns open from the closed position (fully screwed in). You work here when the engine tends to stall at idle or responds too nervously to the initial throttle opening.
Zone 2: from ¼ to ¾ throttle: K57 needle
This is the zone where you spend most of the time on track: pick-up phases, corner exits, mid-range acceleration. The control is the K57 needle, which works inside the DP 267 needle jet: by regulation both the needle jet and the K57 needle are mandatory (they are the only ones allowed). The clip position on the needle, however, is free: the standard one indicated by Rotax is position 2. With the clip in position 1 the mixture becomes slightly leaner; moving it towards the higher positions makes it richer. One notch of difference can clearly be felt in the mid-range response.
Zone 3: from ¾ to full throttle: main jet
At full throttle control passes to the main jet. The jet number indicates the flow: the higher the number, the more fuel passes and the richer the mix. The choice of jet depends on the air density at the time of the race: colder, denser air requires a bigger jet; thinner or hotter air requires a smaller jet. It is the parameter that changes most often on race days.
The regulated components of the VHSB 34 XS
All the internal components of the VHSB 34 XS are regulated by Rotax. Here are the official specifications from the technical regulations:
Needle jet: DP 267
Idle jet: 60
Floats: two, marked 4.0 g
Needle: K57 (the only one allowed)
Needle clip position: free (Rotax standard: position 2)
Air screw: base setting 2¼ turns open from the closed position
Gauge pins: how jets and needle jets are checked
Gauge pins are hardened steel cylinders with a hardness of HRC 60-62, accuracy ±0.001 mm and a certified diameter. They are used to check the internal bore of jets, emulsion tubes and needle jets, making sure the components have not been modified, enlarged or worn beyond the tolerance set by the regulations.
The principle is straightforward: the number identifying each Dell'Orto component corresponds (in theory) to the diameter of its internal bore in hundredths of a millimetre. The 125 main jet, for example, has a bore of 1.25 mm. But often this is not the case and it may turn out to be, for example, a 130 or more. The DP 267 needle jet has an internal diameter of 2.67 mm, but differences can be found here too: gauge pins are the only precision tool for this check. The matching gauge pin must pass through the bore without force and without play: neither too loose nor tight. If the pin of the next larger diameter goes in, the component is out of tolerance and must be replaced.
In race scrutineering, the officials use gauge pins in post-session or post-final checks to verify that the internal components of the carburettor match the homologated ones. In the pits the mechanic uses them during preparation, to rule out wear on components already used for several sessions and to check the actual diameter against the stamped one. At TKS RaceLab gauge pins are part of the standard checks on carburettors before taking a kart on track.
The pins are available in sets covering specific ranges for jets (around one millimetre) and needle jets (around 2.5-2.8 mm). For main jets (range 1.00-2.00 mm) there are similar sets with 0.01 mm steps and the same ±0.001 mm accuracy.
The restrictors in the Micro MAX and Mini MAX classes
The Micro MAX and Mini MAX classes use the same VHSB 34 XS carburettor as the Junior and Senior, with one fundamental difference: a physical restrictor (throttle body restrictor) fitted to the carburettor is mandatory.
This restrictor, official Rotax code 267536, reduces the airflow to the engine, limiting power in a controlled way that is equal for all the youngest drivers. Its dimensions are set by the regulations and it is subject to scrutineering at races.
This difference is reflected in the tuning: that is why the Rotax Jetting App asks for the engine class, and the starting jet changes from class to class.
How to tune the carburation of the Rotax MAX
The correct sequence is always the same: start from the main jet (which depends on the external conditions), then check the air screw, then the needle clip position. The logic is simple: the jet changes with the day, the other two parameters are fine adjustments refined on track.
Step 1: set the main jet according to the conditions
Use the Rotax Jetting App to calculate the starting jet based on temperature, atmospheric pressure and class. It is the starting point: the fine tuning is always completed on track after the first sessions.
Step 2: set the air screw
Always start from the base position indicated by Rotax: 2¼ turns open from the fully closed position. If the engine tends to stall at idle, responds nervously to the initial throttle opening or struggles to hold idle, the air screw is the first thing to check. Make small changes at a time.
Step 3: check the clip position of the K57 needle
The standard position indicated by Rotax is 2. If the mid-throttle response is slow or there is an obvious hole in pick-up out of the corner, move the clip one position towards the higher numbers (richer mixture). If the engine responds flatly or seems short of revs in the mid-range, try the lower position (leaner mixture).
Note for Micro and Mini MAX: with the restrictor the airflow changes, so always start from the jet calculated by the Rotax Jetting App for your class and refine on track.
The Rotax Jetting App
The Rotax Jetting App is BRP-Rotax’s official app for calculating the main jet of the VHSB 34 XS. It is available free on the App Store (iOS) and Google Play (Android) and is the standard tool in Rotax pits all over the world.
It works like this: enter the location (by GPS or manually), the engine class, the air temperature and the relative atmospheric pressure. The app calculates the recommended main jet for those specific conditions.
The key parameters to enter correctly:
Location: use the built-in GPS or enter the altitude manually
Class: Micro MAX / Mini MAX / Junior MAX / Senior MAX / DD2
Air temperature: measured at the track in degrees C, not the weather forecast
Pressure: relative, calibrated to sea level. If your instrument gives absolute pressure, the app has a built-in converter
Base fuel: 95-98 octane unleaded petrol with XPS oil at 2% (mandatory in the Rotax MAX Challenge)
The app does not replace the mechanic’s experience: it provides the starting point, and the final tuning is always refined on track.
The Prisma Electronics AirPro air density meter
For those who want maximum precision in carburation tuning, there is a tool that goes beyond simple atmospheric pressure: the air density meter.
The density meter measures air density, the parameter that most directly affects combustion. Two days with the same temperature and the same pressure can have very different air densities, mainly because of relative humidity. Without a density meter, this variable remains invisible.
The Prisma Electronics AirPro is the reference professional air density meter in karting. It measures in real time:
Barometric pressure
Relative humidity (rH%)
Air density (g/m³)
RAD, Relative Air Density (%) compared with standard conditions
Density altitude
It stores up to 50 sessions, connects via an app to iOS and Android, and lets you export the data to PDF and Excel to keep an accurate history of your race sessions.
In professional teams the density meter works alongside the Jetting App: the app calculates the jet from the data entered, while the density meter provides the real air density at that precise moment, which is sometimes different from the one calculated from temperature and pressure alone.
Reference jets by class
Rotax does not publish a table of fixed jets per class: any Dell'Orto main jet is allowed (a minimum jet may be set for a single event by bulletin). The right jet changes with class, temperature, pressure, humidity and altitude: that is why the reference is always the Rotax Jetting App, which calculates the starting jet for the real conditions of the day.
At altitude the pressure drops and the mixture tends to become richer: the jet must be reduced accordingly. The Rotax Jetting App takes all this into account automatically.
Frequently asked questions
What is the difference between the VHSB 34 XS carburettor and the old VHSB 34 QS?
The VHSB 34 XS arrived with the EVO package, racing since 2015, and in the Rotax MAX Challenge it is the only one allowed: the body must carry VHSB 34 and the XS code. It has specific internal components: DP 267 needle jet, idle jet 60, K57 needle and two 4.0 g floats. It is the carburettor supplied as standard in the Rotax MAX EVO engine kit, not a choice of the mechanic. In TKS RaceLab’s Arrive & Drive we work exclusively with this official configuration.
How many times is the carburation adjusted during a race day?
There is no fixed number: the carburation can be adjusted several times over the same day, even after every session. You intervene whenever conditions change significantly: a noticeable change in temperature, a drop in atmospheric pressure, a significant change in humidity or an approaching weather front almost always require a review of the jet. In TKS RaceLab’s Arrive & Drive this procedure is carried out systematically: jet, air screw and needle position are reviewed before every session.
Do the Micro MAX and Mini MAX classes use the same jets as the Junior and Senior?
No. Micro MAX and Mini MAX are fitted with a mandatory physical restrictor (Rotax code 267536) on the carburettor, which reduces the airflow to the engine. That is why the starting jet must be calculated with the Rotax Jetting App by selecting your class.
Is the Rotax Jetting App free and does it work on all smartphones?
Yes. The Rotax Jetting App is free on the App Store (iOS) and Google Play (Android). It uses GPS to detect the location automatically and can also be used with manual altitude entry. In TKS RaceLab’s Arrive & Drive we always use the app together with the Prisma Electronics AirPro density meter to set the correct jet before every session.
Arriving at the track and the carburation still needs setting? With our Arrive & Drive you get the kart, the track and the technical support of the TKS RaceLab team, at Lonato, Franciacorta, Cremona and Ala.Discover Arrive & Drive →
VHSB 34 XS carburettor, Dell'Orto jets, K57 needle, the Prisma Electronics AirPro density meter and spare parts for Rotax carburation are available in our shop: tksracelab.com
👉 For carburation you will find Dell’Orto spare parts in our shop: Dell’Orto Main Jet Kit.
Conclusion
The Dell'Orto VHSB 34 XS carburettor is a precise, regulated component and, once understood, manageable with method. Knowing what the air screw does, when to work on the clip position of the K57 needle and how to choose the main jet according to the day’s conditions makes the difference between carburation that works and one that leaves power on the table.
The Rotax Jetting App and the Prisma Electronics AirPro density meter turn that method into concrete data: fewer attempts, more precision, less time lost at every session change. They do not replace the mechanic’s experience: they make it more efficient and verifiable.
Technical data: Rotax MAX Challenge 2026 technical regulations and the official Rotax 125 MAX evo operator manual (2024 edition).
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Read also: Kart axles: complete guide to choice, setup and replacement → · Rotax MAX NGK spark plug: how to read it and when to change it → · Kart wheels: complete guide to materials, sizes and choice → · ROTAX Senior vs DD2: differences and who they are for → · Mojo D5 tyres on the Rotax MAX: pressures, management and wear → · Chassis and engine: how they work together on track →
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Author: Sergio, TKS RaceLab | Published: 10 June 2026 | Updated: 4 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.
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