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What Is The Top Speed Of The Rotax Max 125 Engine?

If you have done any research into the world of karting, you would have heard of Rotax before. Rotax make the most well-known kart engine and runs their own competitions, exclusive to their customers. The conclusion of these are the Rotax world finals, hosted in a new location each year.

So, what is the top speed of the Rotax Max 125 engine? The top speed of a Rotax Max 125 engine depends on various factors and variables. Under perfect, unrestricted conditions, the Rotax Max 125 engine can reach 87 mph (140 km/h). It can also accelerate from 0-60 mph (0-100 km/h) in just 2.5 seconds!

There are, however, many factors to take into account when looking at the top speed and acceleration of the engine. In this article I will be talking about what affects the speed of your Rotax kart. We will also be looking at what makes Rotax engines unique and why they are so popular.

A Brief History Of Rotax Engines

Rotax kart engines are a sub-division of BRP; a Canadian company better known as Bombardier. BRP has been in the engine making industry for 90 years. You may also recognize their engines on various other vehicles, such as Sea-Doo jet skis, Lynx Snowmobiles, ATVs, motorcycles, and even some light aircraft.

The modern version of your Rotax kart engine, specifically the Max 125 engines were introduced in 1997. The Junior Max class was introduced shortly afterward in the year 2000. The popularity of these engines grew quickly over a short period due to their reliability and easy maintenance.

The Rotax engine is constantly being worked on and improved year-on-year in order to keep up with their engine manufacturing rivals. Due to the relative simplicity of a kart engine, technology is rather limited, but there is always a way to make small improvements in your engines.

Some Rotax engine alumni are Kimi Raikkonen, Max Verstappen and Jenson Button to name a few. Having such big-name drivers representing their engines has massively helped Rotax in recent years, especially with the development of their customer racing series.

Why Is Rotax So Popular?

Rotax is a popular engine because they run an exclusive racing series for drivers who run their engine. This is the Rotax Max Challenge. The RMC is a racing series with a generalized set of regulations which most karting tracks can run. They are approved by the CIK-FIA. Running this series allows for the drivers from that specific championship to qualify for the Rotax World Finals.

The Rotax World Finals are hosted each year in exotic locations around the world. Recently the finals have been held in countries such as Italy, Malaysia, and South Africa. The Grand Finals see 360 drivers from 60 countries around the globe attend the event each year.

This series has become hugely popular, and become somewhat of a Karting World Championship which most drivers will aspire to partake in. A huge benefit of this series is that all Rotax engines are sealed, preventing cheating and engine tampering.

This creates an even playing field and eliminates some of the disadvantages caused by a lack of finances. In addition, the smaller RMC championships act as a ladder system, which promotes talented drivers who race clean and keep to regulations.

In the Grand Finals, each participant uses the same engine, tires, and chassis, giving each driver an equal chance of winning the competition. This competition is where all the up-and-coming young talent compete against each other.

What Are The Engines For Each Class?

Rotax allows for different classes depending on age and skill level. This allows anyone to start off and enjoy karting. These engine classes also have their own categories in races. This makes it easier for new drivers to know where they need to start.

The amount of horsepower may seem small, however, do keep in mind that the total weight of the kart plus the driver is between 170 and 200 kilograms. This means the power-to-weight ratio is extremely high and can make these karts incredibly fast.

125 Micro Max

These restricted engines are perfect for kids to start karting. With 9.5 horsepower and a maximum of 7000 RPM, this engine is small and restricted to keep the speeds low enough that any mistake made by the driver is forgivable and easy to correct. Being a kart for kids with no experience, the top speed is restricted to around 20-30 mph.

125 Mini Max

The mini max is an engine built for kids who have previous experience in karting. This engine has an increased horsepower of 13 and can run up to 7500 RPM. The top speed of this kart will only be slightly higher than the micro max at around 40 mph, as it is still designed for kids.

125 Junior Max

The next step is a junior class kart. These karts are the smallest class used in international competition and this class runs for drivers between the ages of 11 to 15 years old. These karts have 23 horsepower and can run up to 8500 RPM. They can hit a top speed of around 60 mph.

125 Senior Max

These are the most common adult race engines. This class can be entered by anyone over the age of 15. These engines have 30 horsepower and can run at a massive 12000 RPM.

These engines can be upgraded from the 125 junior max class by installing an electronically controlled power valve, which is where this unit gets its performance boost from. This is the top level of non-gearbox engines. The top speed of this engine is around 75mph.

125 Max DD2

This is the largest engine built by Rotax, putting out 34 horsepower. DD2 is an abbreviation for Direct Drive 2. This means that the engine has a 2-speed gearbox which is operated by shift paddles behind the steering wheel. The other factor that makes this engine unique to the others is that it has a chainless drive system.

This engine is for drivers who are experienced in karting and looking to go even faster than senior class. It also requires some extra mechanical knowledge and maintenance. The 2-speed gearbox allows the engine to always remain within the powerband.

How Do I Maximize My Kart’s Engine Speed?

The top speed of an engine means very little in the bigger picture. Most kart circuits will not have a straight long enough to reach that top speed anyway. Even if the straight is long enough to reach that top speed, if you set your kart up to max out on the straight you will be losing too much time on the rest of the circuit.

This means you will most likely end up halfway down the leader board anyway. As an example, if you look at Formula 1 car speed traps, the cars with the highest overall top speed are not always the fastest in terms of lap times. That being said, having a higher top speed than your opponents can help you in overtaking.

Rotax engines can be tuned to achieve an optimum speed based on the circuit and the weather conditions using several variables which I will cover below.


Firstly, you will need to get your gearing right. Rotax engines have one gear or sprocket, and therefore can either be tuned towards a higher top speed or faster acceleration. The larger the sprocket (number of teeth) the faster your kart will accelerate. However, this will be at the cost of losing top-end speed.

The same rule applies vice versa, too. Having high top-end speed by using a smaller sprocket will cost you acceleration out of the corners. Gearing is mostly determined by the circuit, and you will find that most, if not all the drivers will be running the same sprocket at your local circuit.


The next setup variable is your engine’s fuel mix, otherwise referred to as jetting. The jetting determines how much air is allowed into the combustion chamber to mix with the fuel. You can adjust your jetting inside the carburettor.

Jetting is a very sensitive element of your engine setup, and can be affected by track and weather conditions. You will find that your ideal jetting is different in summer and winter for example. This is also why jetting can always be adjusted to find an ideal balance.

A ‘lean’ mixture is when there is less fuel mixing in the engine. This can be achieved by using a smaller jet and will give you more power in the higher rpm range. In simpler terms, higher top speed, slower acceleration.

A ‘rich’ mixture will do the opposite; more fuel will be used in the mixture. This is achieved by using a larger jet and will give you more power in the low rpm range. Quicker acceleration out of corners, but a lower top speed.

Naturally, you want to find a balance between the two. You can normally tell if the jetting is wrong from a number of signs. Your kart might bog down going out of a slow corner, which is caused by a lean mix, where fuel isn’t flowing in fast enough to combust. On the other hand, your kart might struggle and misfire on the straights if the mix is too rich.

Another easy check is to inspect the spark plug after running the engine for about 20 to 30 minutes. Normally if the engine is running too rich, you will find some residue of fuel and oil on the plug. Ideally, the plug should be dry.

These are always dependent on the track and the weather conditions. Some circuits will have tight, technical sections that require a lot of acceleration, whereas others have fast flowing corners and long straights that will benefit from a high-top end speed. It’s all about what suits the track.

Besides these, there are some smaller details which can give you some advantage, however, they are much more technical and do not offer as much of a performance boost as these two set up elements. If you get these two sections of your set up spot on, I can assure you that your kart will be running as close to optimum performance as is legally allowed.


In addition to this, proper and regular maintenance on your entire engine unit can help you to keep it at peak performance. Here are some regular maintenance checks you should be doing to keep your engine running smoothly:

  • Clean the carburettor
  • Check your exhaust
  • Clean your clutch and replace if necessary
  • Replace oil regularly
  • Check and clean the air and fuel filters

Final Thoughts

The Rotax Max 125 engine can hit almost 90 mph under ideal conditions and on a long enough straight. It can also accelerate just about as quickly as most modern supercars when tuned correctly.

However, under normal race conditions, you likely won’t be reaching these speeds as you will be looking for the ideal balance based on the circuit you are driving on. Remember, you are looking for the fastest lap times, not for the fastest overall speed.

I hope that this article has helped you to learn more about the amazing world of Rotax engines and that you will have a better understanding of how to squeeze out an extra couple of tenths from your lap times by finding the correct balance for your engine.