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Rotax Carburettor Explained

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.

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Know your Tyres



Inconsistency between batches of tyres causes much consternation and muttering with the most recent victim being Michael Simpson at the Larkhall Super One. “We were going alright until we put the race tyres on,” said Andy Cox as he and Michael consoled themselves with a Scottish breakfast. He isn’t the only one it’s happened to and it’s by no means limited to the Rotax classes and Vega tyres, as European KF3 Champion Alex Albon oon Dunlops thought he was at a big disadvantage in his heats. Carlo Forni demonstrates the handling effects of different compound tyres with the same carcass on page 82.

So I asked Jason Parrott, team manager of Max team Tim Parrott Motorsport, whether the situation is really as bad as all that.

“The junior tyres, the SL8s, are OK, they don’t change much, although some people do complain. The SL6s (Senior) are the worst problem, the balance of the tyres changes dramatically, there’s a big slide then a big understeer. The kart won’t stop, and at the middle of the corner the kart drifts. Once we tested four sets at PF with Tom Armour, and there was half a second between the best and worst set.”

“At Larkhall for the Super One the tyres were mostly OK, then at PF for Kartmasters the following weekend we did some testing on the tyres from Larkhall and then the new tyres were only as quick as the 120-lap old ones! It shouldn’t be possible and there is definitely something not right. With all three drivers saying the same thing there must be something going on.”

Jason believes there needs to be some sort of code on the Vegas as there is on Bridgestones so people know what they are getting.

Inconsistent tyres aren’t a new issue though. Alex Zanardi was an early test driver for Vega and in his autobiography he recounts a batch that were particularly good. This was a batch that [Mr Vega’s] severed finger had fallen in to! So in my house a decent set of tyres are referred to as having a juicy bit of finger in them. Who knows what might have fallen in to the bad batches! Although I would imagine that the processes are a big more controlled now…

However, tyres and compound mixing is chemistry and a lot of work goes into making sure things don’t interact with each other when they shouldn’t. The make up of race tyres is a closely guarded secret but in general tyres are made from:

  • Natural rubber, or polyisoprene is the basic elastomer used in tyre making
  • Styrene-butadiene co-polymer is a synthetic rubber which is often substituted in part for natural rubber based on the comparative raw materials cost
  • Polybutadiene is used in combination with other rubbers because of its low heat-buildup properties
  • Halobutyl rubber is used for the tubeless inner liner compounds, because of its low air permeability. The halogen atoms provide a bond with the carcass compounds which are mainly natural rubber. Bromobutyl is superior to chlorobutyl, but is more expensive
  • Carbon Black forms a high percentage of the rubber compound. This gives reinforcement and abrasion resistance
  • Silica is used together with carbon black in high performance tyres, as a low heat build up reinforcement
  • Sulphur crosslinks the rubber molecules in the vulcanization process
  • Vulcanizing Accelerators are complex organic compounds that speed up the vulcanization
  • Activators assist the vulcanization. The main one is zinc oxide
  • Antioxidants and antiozonants prevent sidewall cracking due to the action of sunlight and ozone

Tyres are changed from a gloopy mess to something that stays in shape by vulcanization where sulphur or other curatives are added. These additives modify the polymer by forming crosslinks (bridges) between individual polymer chains. So as you can see there are several stages where the manufacturing can go slightly wrong and the resulting tyres won’t work as expected.

However, a lot of the investment into pharmaceuticals goes into making a consistent formula, and medicine is 1) life or death, and 2) often very expensive. It might be that the tyre manufacturers have found the best balance they can between price on the one hand and consistency and quality on the other. However, if a company can offer an improved product we need to go with that.

Next year of course, the Max classes will all change to the Mojo tyres. They would have been changed for this year, but PCA-free tyres are being encouraged for all applications by the EU, and the 2011 Mojos will be PCA-free so the ABkC and MSA decided to avoid having two changes in two years.

In Jason’s experience in Euromax, these seem to be a lot more consistent so hopefully Max racers will have better experiences once the Mojo tyres are phased in. In testing by the ABkC, performance has been found to be similar.

“The tyres are so important that if they aren’t right, changing the chassis, the axle and all of that makes absolutely no difference. It does give people an excuse though!” was Jason’s final point.

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Warming up an engine

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Rotax engine start up

The Rotax Max has been around for more than sixteen years now. Due to the failure of classic 100cc racing and the subsequent CIK KF Classes the Max is considered by many to be the pinnacle of Karting internationally.

Of course this success is gratefully received by the members of the industry that are involved and indeed by BRP-Rotax, the manufacturers. However in some respects we have to be wary that the classes do not become victims of their own success, the concept of the Rotax Max has been copied but never equalled.There has been much talk over the past year about the “New CIK KF engine”, this will eventually happen but it is doubtful that it will happen at all before 2016 if then. In the meantime the stability of the Rotax max classes remains undiminished, indeed stronger than ever.An area often avoided by the trade and industry is the ability of the driver, in some circles referred to as the nut behind the wheel! Having been involved with the Max since the beginning, I stand by what I first said. Anyone can drive a Max but very few can drive one really well. In spite of gradual development over the years making the engine more user friendly, there is a vital element in the mix that makes a kart consistently fast; the driver.

These days at the highest level there are big teams with skilled managers, mechanics and driver coaches; that is all well and good but could be the thin end of the wedge that leads to this great product being a victim of its own success. What about the entry level club driver that wants to enjoy his racing with a reasonable level of success at a reasonable price. The start of driving a Max well starts at home with the preparation of the Kart for your weekends racing, a clean and well prepared Kart will give you less work at the track which enables you to concentrate on final set up changes and hone your skills for a successful outing. Make sure the whole machine is perfectly clean, not just spit and polish, you may easily fins a crack in your chassis, axle or exhaust that could spoil your weekend if gone unnoticed. Get yourself to the circuit in good time, sign on, pay your dues and be sure to be ready for the first practice session. A lot of people stall the engine on the dummy grid repeatedly there is a simple method to ensure a smooth and trouble free get away. Make sure that the throttle is fully closed and not held open at all, this does not include adjustment of the idle screw. The choke need only be used at first start up and even then only on the coldest of days. Most important of all is to be sure that the fuel is fully blown through and that the Carb is full of a fresh petroil mix. Turn on the ignition, press the start button, as soon as the engine fires apply a very little throttle, the engine will gather RPM and gently engage the clutch, return the choke to off before going on track.

The practice of starting the engine on the stand to warm it is not good for the engine internals, particularly the balance shaft and gears as well as the piston, the engine will warm up more uniformly and efficiently and so will the tyres if you take at least one lap gently before you give it the beans! If the weather is really cold, we fill the radiator with hot (not boiling) water and cover the engine with an old padded jacket. The engine will then be closer to working temperature before you even start it up. If you only have 3 laps practice before racing begins, this can be a really good idea. Imagine your engine has been stored overnight in your van with no heating in winter the engine has Anti-freeze (we hope), under these conditions the engine core temperature could well be below freezing point, poor little engine! If you take the above advice regarding hot water, whatever you do, don’t forget to drain it out again before you load up to go home. A frozen engine will almost certainly wreck the cylinder and damage the water pump seals.On track, be smooth, brake as near as possible in a straight line, accelerate smoothly, don’t snap the throttle open and you’ll soon be up there with the best!



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The MAX Column

MAX Column LogoI have been having a busy time of it recently, two Rotax Super 1 meetings in a fortnight followed by a visit to the BRP Rotax factory in Austria.

Having landed at midday on Thursday at Gatwick, I collected my car and drove straight to the tunnel on route to Le Mans for the 2 stroke international 24 hour. I am currently sitting in the paddock at Le Mans where the start of this crazy race is today at 3pm. The entry is the strongest for the past few years with the vast majority choosing Rotax Max power. The regulations allow the use of two engines during this race, but many will complete the whole event on just one.

There is no doubt that engines wear less when they remain in one heat cycle and the internals of MAX engines that I have built following this race are always in exceptionally good condition. It is quite extraordinary that engines that are built exactly the same as a top level sprinting unit go on to set their fastest laps in the early hours of the morning when they have already run flat out for at least twelve hours. The 24 hour race may be considered by some to be a slow endurance test. Please do not pass judgement without the knowledge that some of the most respected drivers in the world have contested this event since its inception in 1986.

The Rotax max is the ideal product for long distance racing, over the years competitors have come and gone from most of the rival engine manufacturers, today there are none, with the exception of a handful of KF teams that run in a separate class. The Rotax runs in the most popular classification, all rivals having fallen by the wayside through lack of speed, reliability or both. I am here again this year with an involvement in just a couple of teams, there was a time when I supplied engines to as many as ten different entrants, however times have changed and I am at ease with the situation, it is true to say that tonight will be one of a lot of noise and very little sleep.

The Rotax factory visit was a very good couple of days. I was fortunate to be invited to attend as the purpose of the visit was their annual World Distributor meeting. We began with a factory tour which was amazing. The Austrian base is the engine plant for all BRP Rotax products including Aircraft, Seadoo, Skidoo, Johnson, Evinrude, Can-am and other brands built under licence for top names in the industry. BRP Rotax are a stand-alone company that is no longer part of the Canadian based manufacturer Bombardier which is famous for products including executive aircraft and rolling stock that includes the Eurotunnel trains. There is not competition between Bombardier and BRP as the “RP” stands for Recreational Products.

The factory are constantly looking to improve the delivered product. It is extraordinary that the Max engines has been on the market for sixteen years now and there are still the very earliest engines in use today. Over the years of course there have been evolutions, but none have written off the earlier versions. Such a long product life has never been seen before in the world of Karting. The product line has never been in a stronger position in the World market. It is only a disappointment that we, in the UK have no support for the DD2 category. The class is growing rapidly abroad and is offering a very exhilarating alternative to the wildly successful Senior Max.

I have just witnessed one of the best races I have seen for years, Les Vingt-Quatre Minutes De Mini-Kart. A 24 minute race for Micromax, a full grid were all within a couple of tenths in qualifying and the race settled into a battle royal between five with no quarter given. All five were abreast on the straight on the last lap. I knew it was not the UK because there was no contact, just good clean racing. I know we benefit from slip streaming, but there does not need to be contact. A contact lap will never be the fastest potential lap. Race craft is a skill some of these children seem to have naturally, it is great to see and impossible to teach!

The main event at Le Mans starts soon, as I said before there are many top name drivers competing, a name known to many that was good to see is Allesandro Piccini, multiple World Champion in the KZ category and here to add the 24 hour to his extraordinary CV.

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How to Keep your Engine Cool


The heat of the summer is upon us and can take both man and machine by surprise. Most of us enjoy the warmer weather and here are some tips to ensure your Rotax enjoys it as much as you do.

From the Rotax Max engine point of view, an engine in good condition will easily withstand any heat the UK weather can throw at it. The ideal working temperature for all classes of Max is between 50 and 60 centigrade. If under extremely hot conditions your engine exceeds this, there is really nothing to worry about from a reliability standpoint. In hotter Mediterranean or Middle Eastern countries engines frequently run at 80 + degrees with no ill effects. It’s true to say however that the cooler you can keep the engine in hot weather the better the carburetion will be. The engine needs a good core temperature to get the exhaust gas temperature up to efficient levels. This is why engines don’t always perform equally when the time from pits to racing is very short.

Most engine builders will fill serviced engines with coolant mix. The most basic type is usually blue, this gives perfectly good frost protection and a basic level of corrosion inhibitor. The better automotive coolant is pink which gives a higher level of corrosion protection and is better still for water pump lubrication. Unfortunately both these types of anti-freeze don’t help the engine run cool. Plain water will definitely run cooler than anti-freeze, distilled or de-ionized water will run cooler still and has a less acidic effect on the internals of your engine. All good news then? Not quite, first you lose the lubricity of a good coolant and second you must remember to drain and refill the system before frost. There are also coolants that enhance cool running. This can be a win-win situation as some of them also provide adequate frost protection as well. These coolants are usually more expensive than regular types. A couple of brand names to look for are Ice cooler, which is green and Water wetter which is pink. Before using these products read and adhere to the insructions.

In very hot weather there are other areas that come under extra load such as transmission and gears. It is generally true that hot weather brings more grip to the circuits and so the chain and sprockets need to be in as new condition and well lubricated. Since the introduction of the clutch O ring there is very little evidence of grease getting into the clutch. If you want to be sure, take the drum off, clean the inside with brake cleaner, put the smallest smear of grease on the inside of the sprocket bore and re-assemble having checked that everything is dry and clean. While the chain is off lubricate it well with a high quality chain lube and allow the propellant to burn off before refitting it. This will ensure good lubrication for the duration of the race and cut out the risk of a scrutineering issue with a wet clutch. Only chains and sprockets in perfect condition provide efficient transmission. People are always searching for fractions of a horsepower and then fit used sprockets and chains that rob ten times that amount.

It is worth experimenting with a richer fuel/oil mix. The Rotax recommended 50:1 or 2% is adequate for engine reliability but a mix of 3 or 4% can help to keep the engine cool and constant for the whole race. An interesting by-product of this is that the carburetion also becomes easier to jet. As a matter of good maintenance, check and change the gear oil frequently. The recommended service interval is five hours. Why not check it before each race meeting? We’re always surprised by how many engines come in for service with very little oil in the gear-case. Most customers admit to not checking it since the last service.

In hot weather the largest single loss of performance is within the driver! Stick with cool mineral water. When you realise you desperately need a drink, it’s too late; you’re dehydrated. Continually sip water and take a good couple of mouthfuls before putting your helmet on to race. It’s a good idea to leave a small gap at the bottom of your visor which will allow a little cooling draft to keep you alert. Don’t forget your helpers need fuel and water as well!

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Has Rotax Changed Karting?


I spent a couple of days at the opening round of EuroMax at Genk. It’s clear that British teams have a good share of the existing market…

There are also British-based drivers who race in EuroMax who choose not to race here in the UK. I am not going to elaborate here on the potential reasons for this, I will leave it for you to speculate. However beware that your speculation is not suicidally depressing. EuroMax is alive and well. Under the expert guidance of the Guiedel family of Roland, Lynn and James the whole meeting exudes professionalism and excitement.

Here in the UK we can see used equipment changing hands in the leisure market for less than £1000 while top performing engines can command significantly more. The BRP-Rotax warranty scheme remains unique in motorsport and has been the company’s single most successful marketing tool since its inception more than ten years ago. From the manufacturers’ standpoint the warranty scheme is purely a means to see how well the product is performing out in the market place while benefitting the customer base at the same time. The fact that a competitor at the most basic of non-MSA events can complete a season on a budget of a few hundred pounds is fantastic. The fact that this guy is probably enjoying his sport every bit as much as the well-funded championship competitor is indeed a fact that should not be lost on the powers that be, cosseted in their splendid offices just west of Heathrow Airport.

Our new ABkC chairman seems to be determined to shake up the sport. There are thousands of people out there that love the sport of karting and yet the official licence figures are continuing to drop year on year. At what point will the governing body declare that it can no longer offer its support? There needs to be a hierarchy within the sport that welcomes the newcomer and nurtures the seasoned competitor. Historically we have produced some of the best drivers in the world and continue to do so today. The future of the sport as we know it is here and now, it would do us all well to listen to the wise words of Colin Wright, the new ABkC chairman as well as those on the international playing field such as Roland Guiedel of RGMMC, the promoter of the EuroMax series. The great success of EuroMax should be studied carefully. Its success is effectively removing drivers from racing at the highest level in the UK. Good for them, but not so good for the home market and definitely not so good for the future of the sport. Without the feed of competitors from the home market, where are the next generation of competitors going to come from?

Karting doesn’t have to be too expensive at entry level. The trade need to nurture beginners in order for them to stay in the sport and enjoy it enough to feel that they want spend a little more to progress within the sport. Rotax Max is the ideal if not the only product that can help them to do this. While the sealed engine system has had its critics in the past it is now proven to be the most successful formula the karting world has ever known.

Never before has one model of engine continued in production and at the forefront of completion for so long. The Max is now in its 17th year, and going from strength to strength.

There have been three generations of Rotax kart engine production. The first was a direct adaption of a motor cycle engine – the 256 tandem twin. This 250cc twin-cylinder rotary valve engine destroyed the establishment in Superkart racing and since production stopped, the various clones have continued to dominate this branch of the sport. In the late 1980s, the Rotax D100 landed in 100cc racing like an atomic bomb. Within six months it had swept all before it. Within two years all the regular engine manufacturers had produced their versions to compete against the all-conquering Rotax models of their day.

the Max was introduced in 1997. By this time 100cc racing was already in decline and was all but dead by 2004. Other manufacturers produce a wide range of TaG (touch and go) engines but none have equalled Rotax in terms of longevity or reliability. The success of the Max range of engines has been global and the market is still growing strongly.

Here in the UK, but we have a high proportion of our customer base in the MiniMax and Junior age groups. Rotax Max is continuing to grow here right from the entry level up to the most popular and competitive classes at national championship level.

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Rotax Maintenance


The last thing that you need is unreliability when you arrive at the circuit. For this reason I thought it would be a good idea to devote this column to Rotax Maintenance and checks.

Hopefully the battery has been stored in a charged condition, the regular lead acid battery that is supplied with engines is the Yuasa. These have proved to be very reliable. If the budget is not too tight a spare battery is always a good item to have to hand. If a battery has been left for months in a discharged state it may never recover to its former strength.Low cost equivalent batteries are not an option. The Rotax Yuasa battery shared its dimensions with very inexpensive other brands. These cheaper units are in fact designed for alarm systems and are not shock protected. While the Rotax battery is not a true Lead acid Gel type it does have shock protection between the plates.

This material absorbs the acid and protects the plates from contact with each other. The gear oil should be drained and replaced. With the old style clutch the engine will almost certainly have plastic balance gears. These should have a maximum fill level of 50cc. The late and current type of steel gears should have a maximum fill of 100cc. There is a Rotax approved and supplied gear oil. Alternatively a good quality light gear oil will do it. The next step to happy days is to clean and check the wiring loom. I think it is probably best to remove this from the kart and give it a really good wash. The most important tiny item, often overlooked is the red rubber seal that fits inside the multi-plug connector that attaches to the bottom of the coil. If you are using a coil extension bracket, there are a couple of words of warning. Firstly you need to be sure that the loom is not too stretched from the ignition pick up on the back of the crankcase up to the multi-plug on the coil.

Some of these brackets are just a bit too long so it is worth making sure that their adjustment allows for the loom to be safely attached. It is also imperative that the thick black earth wire from the loom is attached to the engine and not remotely so that all of the earth has to travel through the short earth strap which bridges the coil mounting rubbers. The condition of this earth strap should be carefully checked and replaced if in any doubt. Poor or incorrect connection here can lead to misfires, bad performance and at worst, destruction of the coil itself. A final word of warning in this area is that there is a temptation to shorten the HT plug lead when a coil extension bracket is fitted, this is not allowed within the regulations; there is a minimum length. While in the area of the spark plug, remove it clean it carefully (brake cleaner works well) and connect it to the HT lead. It is easy to check that you have a spark by resting the plug on the cylinder head and cranking the engine.

The way the wiring loom is constructed allows the engine to crank without the ignition on/off switched on. Before making this check be sure that there is no risk of fuel igniting when the plug sparks! The switches can be a source of trouble but this is rare. If the above check is successful then it is clear that all is well. A common mistake is incorrect wiring to the switches. All wires to the switches are on the + or positive side of the loom, there are no negative or earth connections there to confuse you. The thick wires always go to the start button and the thin wire always go to the on/off switch. On more recent looms one of the thin wires is “piggy-backed” off the thick live supply wire, always be sure that this thick wire is connected to the start button. Assuming that the switches are in good order you should now be able to crank the engine over with the spark plug out. Again it is a good idea to ensure that the wiring isn’t tight and pulling on the switch connectors. This is the most common cause of switch problems. Often overlooked is the fuel pick up in the petrol tank.

This is a regular length of fuel pipe with a brass weight or filter on the bottom end. The length and flexibility of this hose is of great importance. The brass weight should sweep the bottom of the tank and the flexibility of the hose will allow it to follow the fuel as the centrifugal force moves it around in the corners. Fuel starvation can be a really difficult fault to diagnose. The carb is very simple, but you do need to know what you’re doing to keep it in good order. If you want to do your own service work, you will need some special tools and a spotlessly clean environment to do it in. I was lucky to be taught how to work on carburettors when I was at school and a lot of that instruction still helps me every day. If in doubt the Carb is something that you should entrust to your friendly local Rotax Service Centre..