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Jetting - What's it all about?


Getting back to basics!

A layman’s guide to the science and why you have no choice if you want to get it right!


We’ve all been there – you spend several sessions tweaking the carb, changing the jet etc and then you have the magic lap, when the engine just seems to be right on song. All is well and you get to focus on your lap time, setting up the chassis, altering the tyre pressures etc. At the end of the day, you put your fresh tyres on and put it all away, looking forward to the race tomorrow.

In the morning, you get your few flying laps before the racing starts and hey ho, the engine is bogging or popping and the jetting is off again. What to do? Up or down a jet and hope that it’ll all be good for the race. Your mate next door says he’s been running the same jet all weekend and his engine is bang on the money. He hardly ever changes it. Someone else says you should be using a different size jet etc etc etc.

It’s all rubbish. Why? Because jetting, like all the other dynamics, comes down to the laws of physics. It’s a science – not an art. Yes there are variables but they are physical ones. Now I’m not going to give you all the calculations and formula that go into working it out properly – that’s not what this article is about. What I am going to do is try to explain in layman’s terms how it works and why things happen so you can make a better judgement and hopefully have a better understanding of the subject.


Jetting, like all the other dynamics, comes down to the laws of physics. It’s a science – not an art.


What’s it all about?

Firstly, we need to understand exactly what it is that you’re trying to achieve. Petrol (gasoline) needs oxygen to create the explosive mixture that is ignited by your spark plug. Like any combustible, there is a specific ratio of petrol to oxygen that gives the optimum explosive mixture. The oxygen is contained in the air. The air/fuel ‘charge’ is chemically balanced and this Air/Fuel ratio is called the stoichiometric mixture (usually abbreviated to “stoich”).

In the simplest terms, our objective is to mix the petrol and oxygen at the specific ratio and deliver that to the combustion chamber for our spark plug to ignite. We need a means to do this - the carburettor, we need petrol - from the tank and we need oxygen. The problem is the oxygen.

Air contains other gasses so we need to work out the amount of oxygen in the air. We do this by determining the air density (how much a given volume of air weighs). From this you can calculate the oxygen content. High air density indicates more oxygen is in the air, so more petrol is needed to maintain the correct air/fuel ratio, whilst lower air density indicates less oxygen is in the air, requiring less petrol to maintain the correct air/fuel ratio. So how do you calculate the air density? This can be done from the temperature, humidity and pressure of the air – you also have to take in to account altitude.


If you want to run at the front then you have to get it right every time. Near enough is just not good enough.

Now the fun starts!

If you have worked out the air density you can work out how much petrol is required to maintain the correct air/fuel ratio. Then you can utilise the variable settings in the carburettor to deliver that air/fuel mixture (the ‘charge’) to the combustion chamber of your engine. Easy? Well not exactly! There is much, much more to it than that. Think about it. The carburettor has a main jet, a needle jet, needle (which is shaped and tapered), pilot/idle jets, venturi, floats etc all of which are available in different sizes, weights and you can alter some of their settings – such as the float weight/height, needle type (different profiles), needle height, idle jet size etc.

The engines, carburettors and parts that we use are not identical – they are mass-produced so there is a manufacturing tolerance. Whilst manufacturers quote a 'standard' spec for engines, and especially sealed racing engines such as the Rotax FR125 MAX, the fact is that no two engines and/or carburettors are exactly the same. In the case of the MAX if you were to dyno them, although the power would be within a small margin similar, the characteristics of each engine will be quite different. As a result, you are unlikely to ever find two engines, for the same given spec that will use the same jetting and needle clip position throughout the throttle range. Add to that changes in the petrol ‘mix’ when you introduce two-stroke oil at various ratios, the differing specific gravity of the plethora of different petrol brands that are available and you should start to see that it’s not just a simple case of using a chart with main jet sizes referenced against air density.

This is why charts and the ‘slide-rule’ type of jetting aids just don’t work. True they might get it right once or twice in the same way that if you are blindfolded and throw enough darts at a dart board, one day you might just hit the bullseye. But that simply isn’t good enough if you’re serious about your engine performing to the best of its ability. It’s just not accurate enough (or accurate at all). If you want to run at the front then you have to get it right every time. Near enough is just not good enough.

The needle profile and the height setting are critical for your low to mid range performance.

One of the biggest mistakes made is the assumption that the needle height has a linear relationship to the main jet. There is a relationship but it isn’t a linear one. The needle is there to alter the flow (and therefore quantity of fuel mixture) that is introduced into the air-stream flowing through the venturi of the carburettor. The needle has a taper – in fact virtually all needles have varying degrees of taper over their length. With the throttle closed, the needle is almost blocking off the needle jet so little or no fuel mixture is flowing into the air stream through the jet.

The throttle slide is generally shaped to allow some air flow when it’s closed and there is a small progression port that allows some fuel mixture to flow so your engine will tick over. As you press the throttle, it lifts the slide and therefore the needle. This allows more airflow and, as the needle is raised out of the needle jet, more fuel mixture to flow past it and into the air stream. The needle position (height) and where the needle tapers, in relation to the needle jet ‘hole’ dictates just how much fuel mixture flows. The needle profile and the height setting are critical for your low to mid range performance. If the needle was all the way up and out of the needle jet, then the full bore of the main jet would be dictating the fuel mixture flow. We get there or rather near to there when running at wide-open throttle (WOT).

Never assume that all engine/carburettor/exhaust combinations are the same – they are not.

There’s more.

The height and weight of the floats and specific gravity of the fuel mixture dictates the height of the fuel mixture in the float bowl and therefore, how much fuel mixture will be sucked through the main jet and needle jet into the air stream. Altering the float height, weight and the specific gravity of the fuel mixture can have a dramatic affect on your overall jetting.


If you want to accurately establish the correct settings for you carburettor, you need to calculate the air density, calculate the specific gravity of the fuel mixture, calculate the fuel mixture flow for a given needle, needle jet, venturi, progression port flow, idle jet flow etc and then repeat it all for the various throttle positions (remember the needle is tapered).  Simple software that is available either doesn’t make all the calculations, assumes that all engine/carburettor/exhaust combinations are absolutely identical or worse, both. It just isn’t that simple and we haven’t even discussed the temperature range of spark plugs, dirt/moisture in the fuel, carb ice or driving style!

So, in summary:

  • Never assume that all engine/carburettor/exhaust combinations are the same – they are not.

  • Never assume that what works for one will work for all.

  • If your mate is winning do you think he’d really want your engine to be performing the same or better than his? If he isn’t winning, why listen to him?

  • Never assume that the weather (and therefore the air density) will be a constant – nature will prove you wrong time and time again.

  • The laws of physics are universal. The calculations are the same the world over.

  • Getting the best performance from your engine across the throttle range depends on a lot more than the size of main jet.

  • Charts and slide-rules can never be accurate (they assume that all engines and carburettors are absolutely identical). Manufacturers who provide them will always play on the ‘safe’ (rich) side of the equations.

  • There is only one way to set up your carburettor accurately. The physics that apply and the way in which they work are all you have.

  • Engine builders, ‘mates’ and paddock ‘gods’ who’ve always done it that way (finger in the air?) and don’t believe you need accurate software to work it out either have brains more powerful then mere mortals or don’t understand carburetion and as a result, are talking rubbish!


Knowledge is power. Correct carburettor setup is engine-power.

Why we (and the thousands of customers who use it all over the world) believe Jet-Tech is the only product available that works and as a result, why Jet-Tech is the worlds leading product!

We have achieved this by using a Dynamic Simulation Model© to precisely model how the single-choke and diaphragm carburettors work and by incorporating a unique calibration facility into all versions of Jet-Tech software. Unlike other products on the market, Jet-Tech uses its unique calibration to exactly match your individual engine/carburettor/exhaust combination and setup.

There are over 64,000 (sixty four thousand) lines of code in Jet-Tech, the bulk of which are performing the calculations. Do you really think we would have spent over 12 years of continuous development and done all that work if it wasn’t necessary?

Jet-Tech has a unique and copyright feature to enable you to calibrate your barometer accurately, allowing for altitude and pressure variations. The software uses data from internationally certified weather monitoring and meteorological organisations to establish the correct altitude and pressure. It then relates  this to your barometer reading (taking in to account altitude at the location of calibration) and then compensates for any discrepancy (tolerance) in your barometer.

These unique features together with other things that we don’t reveal for commercial reasons enable us to achieve an accuracy of within 0.299%. No other vendor does or can claim this level of accuracy.

So do you need software?

If you want to get the best from your unique engine/carburettor/exhaust combination, each and every time you go to the track, how else are you going to achieve it? You can guess and there may be times when you get it right but each time you don’t get it right, it’s costing you money, wear on your engine/kart, wasting tyres and valuable track time. Of course you have a choice but why is it do you think, that professional racing teams across all forms of motorsport use software to set up their engines, their chassis and record chassis data?

Do you think it’s expensive?

Think about it. How much track time do you spend trying to get your jetting right. Every lap taken is putting wear on your engine, chassis, bearings, brakes and tyres. Jet-Tech costs about the same as a set of tyres but it’s a one-off cost. If you only saved one set of tyres over a season of racing and testing, it’s easily paid for itself. How many (if any) other components or pieces of equipment that you purchase, pay for themselves within a season?

Here's what a couple of our customers say......

"It's not much more than a set of tyres and it'll last a blooming sight longer. Think of it this way. If you don't buy it, then the set of tyres you buy are a waste because you won't get the best out of your equipment. It'll last for years and when you equate it out, it's minimal (cost)."

"Best money I have spent in karting yet, period!”

         ......more customer comments here.

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