Jetting - What's it
Getting back to basics!
A layman’s guide
to the science and why you have no choice if you want to get it
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.
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.
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.
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
If you want to run
at the front then you have to get it right every time.
Near enough is just not good enough.
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
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.
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
fact is that no two engines and/or carburettors are exactly
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.
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.
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.
the float height, weight and the specific gravity of the
fuel mixture can have a dramatic affect on your overall
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).
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:
that all engine/carburettor/exhaust combinations are the
same – they are not.
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?
that the weather (and therefore the air density) will be a
constant – nature will prove you wrong time and time
The laws of
physics are universal. The calculations are the same the
best performance from your engine across the throttle
range depends on a lot more than the size of main jet.
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
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.
(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
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.
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
So do you need
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
Do you think
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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