Jetting from the bottom to the top! by Bill Givens
To properly jet your carburetor the carb must be clean and with sound gaskets, 
seals and o-rings. Vent tubes must be clean and free flowing. Disassemble 
and clean the carb and all the passageways with carb cleaner and compressed air.
The engine must be in sound mechanical and electrical condition with no air leaks. 
So… do a compression check along with a pressure check to make sure you have 
a good foundation. A new spark plug with the correct gap and properly set ignition 
timing is also helpful. You can tune till you're blue in the face, if you don't have a clean 
carb, with a mechanically sound engine as a baseline.

Both Mikuni and Keihin’s recommended jetting procedure is to start from the 
bottom and proceed to the top. A properly set float level is critical to all circuits 
in the carburetor and it must be set first. A simple and fairly accurate method to 
set the float level… is with the float bowl removed, tilt the carb body to about a 45° 
angle and watch the float needle. Most have a spring-loaded needle… 
what you are looking for is the point where the float tab just meets the needle 
valve, without compressing it. Continue to tilt the carb to the point where the 
spring just barely touches, and then look at the float casting marks found on 
most floats. With a properly set float the marks will typically be parallel to the body 
of the carburetor. This point is also the point to take your float level measurement 
and with the float in this position the distance should equal the required measurement, 
per the manufacture’s specifications. If not… carefully bend the tab that the float needle 
rides on to get the correct level.

Please remember that we tune each of the carburetor circuits based on throttle position 
and not RPM. We also go from the bottom to the top, because to varying degrees there 
is a culmination/overlap of mixtures from one circuit/position to the next one. If these 
overlaps were not present and correctly tuned for… you would experience flat spots or 
bogs and/or surges as you open the throttle.

Idle to about ¼ throttle opening is primarily controlled by the pilot circuit and this fuel/airflow 
continues all the way to WOT… and thus affects the whole jetting spectrum. This is one 
of the major reasons that you jet from the bottom to the top.
1/8 to about 3/8 is controlled by the slide valve cut away.
¼ to about ¾ is controlled by the needle jet and jet needle.
5/8 to WOT is controlled by the main jet.
Note how each circuit overlaps!
An important point to note is that if a particular circuit is either too rich or too lean… 
the amount of overlap can shift up/down, shrink/expand the range that it should normally 
control. Due to an engines design characteristics, rider driving technique and a multitude 
of other variables… a particular circuit may need to be richer/leaner than it would be in a 
different situation. Then an adjustment would have to be made on either side to make the 
transition smooth.

The transition from one area to the next doesn't have a Red or Green light…it has a 
blinking yellow, and just like in driving, you need to proceed with caution. If you start 
from the bottom and go up… you’re tuning for these variables as you open the throttle… 
allowing the next step to be more defined. Remember… that’s what we’re looking for… 
smooth transition from bottom to top, with the correct mixture ratio. Then as you roll 
off the throttle you'll have a smooth transition back down the range also. This is just as 
important! A lot of two stroke engines start to seize shortly after you come out of the 
throttle, not when you are WOT! Why… because when you shut off the gas, you also 
shut off the oil, and after a long WOT run you have built up a lot of heat, and you need 
the gas/oil mixture to protect your piston/cylinder walls.

Now that we have the float level set and have a basic understanding of how the circuits 
affect each other… let’s just do it!

The pilot circuit is controlled by the pilot jet and air/fuel adjusting screw. If the 
adjustment screw is on the engine side of the slide it controls the fuel, and if it’s on the 
air cleaner side it controls the air. Most MX style carburetors have the adjusting screw 
on the air cleaner side, so it controls the airflow. A simple way to look at this circuit is 
the pilot jet controls the fuel and the air adjusting screw allows you to add the appropriate 
amount of air to get the proper air/fuel mixture.

To tune the pilot circuit… lightly seat the adjusting screw by turning it clockwise till it 
just touches the seat. Don’t tighten it too snug or you might damage the seat or the 
taper on the end of the adjusting screw. Now back it out to a base line setting of 1-½ turns. 
Warm up the engine to a normal operating temperature. Adjust the idle speed adjusting 
screw so that you are about 500 RPM higher than your normal idle speed. Make a mental 
note of the air adjusting screws location, and slowly turn the adjustment screw in, 
and then turn it out in 1/8-turn increments, until you have found the highest idle RPM. 
Go slowly and let the RPM stabilize. Go back and forth a couple of times so that you 
get it spot on. If the circuit is adjusted properly you will have a smooth idle and the 
transition from idle to about ¼ throttle will be also be smooth, with no hesitations or flat spots.

Note how many turns it took on the adjusting screw to get your best idle speed. If you 
had to turn the screw more than one turn out from the baseline of 1½ turns, then the pilot 
jet is too large and you need to go down one size on the pilot jet. Now go back through 
the adjustment procedure again to get the adjustment screw back to the base line setting 
of around 1½ turns out, when you have the best idle. Most adjustment screws have a taper 
that works best between 1/2 to 2 1/2 turns. Using this method allows you to make minor 
air adjustments to compensate for small changes in weather conditions at the track and 
still be in the working range of the air screw taper and spring.

The next rung up the ladder is the throttle side cut away, which controls the amount of air 
allowed to flow through the carb at throttle opening in the 1/8th to ½ throttle range with the 
most affect in the 1/8th to ¼ range. Typically… slide adjustment are not required, but… 
you might run in to a situation where you need to change to a leaner or richer cut away. 
The higher the number of the slide the more air it will flow within it’s operational range. 
Slides are numbered in millimeters of cut away at the closed throttle position. A number 
6 slide has a 6 MM cut away and a richer number 5 has a 5 MM cut away. While you 
can typically adjust the pilot and slide with the engine running and the vehicle still… 
a good on the track test is also recommended. Accelerate out of a slow, first gear 
corner to give it a good test. The slide works in close conjunction with the pilot circuit 
to allow you to transition to the next tunable area, which is the jet needle and needle jet.

We want a smooth transition from the throttle slide to the main jet. This transition is 
controlled by the needle jet and jet needle. When completed, we want the jet needle’s 
(more commonly called the needle) clip to be in the middle position, if possible, for 
our base line setup.

The needle is tapered, and has groves cut in one end, to hold a clip. This clip allows you 
to change the length of the needle. There are also small, thin spacers available, that will 
let you set the needle length even more precisely, by placing them under the clip.

The needles length, diameter and taper, in conjunction with the needle jet control the 
rate of fuel flow in each position of the needle from a little less than ½ throttle to a little 
more than ¾ throttle.

Needle jets come in a couple of different styles and of course numerous jet sizes. 
The provide additional avenue to fine tune your mid range. Some tuners find it more 
convenient to replace the needle jet with one size richer instead of raising the needle. 
This way you don’t have to take the top off and mess the springs, etc. 
This way your needle will stay in middle clip setting all the time. Different strokes for 
different folks!

Keihin and Mikuni use a different system for numbering their needles and you need 
to become familiar with whichever carb you are running. Give Sudco’s web site a hit 
and look through their needle info to find out what might fit your particular situation. 
You can get the mid range perfect if you take your time and have a selection of needles 
and needle jets, which by the way, is sometimes called an emulsion tube. Sudco also 
sell a tuning manual for Mikuni that it great… for about $12.

Ok... let's spend some time talking about the big three in needles!
The needle diameter controls the lower end of the mid range jetting... just above the 
pilot jet/slide area, and you feel it in the ¼ to ½ throttle range. The setting of the needle 
diameter is critical to the engines lower mid range power and drivability. When the 
needle diameter is too large or lean… the engine could have a “lean bog” just as it 
starts to pull well. When the needle diameter is too rich, you'll normally suffer a “rich bog” 
in the ¼ to 1/2 throttle position.

Ok…what’s the difference between a rich bog and a lean bog… A lean bog is a sudden 
loss of power with the characteristic boooooooooooowwwwwwwwww exhaust and intake 
sound, and if it’s way too lean… you will not be able to pull through it without letting off of 
the throttle a little bit. A rich bog has a much more mellow sound along with a blubber, 
stutter feeling and you will typically pull on through the range… in time. It should be noted 
that either one of these situations can be caused by your fuel delivery system or by clogged 
vent tubes.

Moving on now to the overall length of the needle. On most needles there are 5 clip positions. 
The top clip setting is called the #1 position and it’s the leanest. The length setting covers a 
wide range of your mid throttle jetting, with an emphasis at ½ throttle. The setting of the needle 
length is critical to the engines mid range power and drivability… very similar to the diameter…
just a little more up in the range of the throttle position. When the needle is too long… the engine 
could possibly have a “lean bog” in the mid range, and conversely, when the needle diameter 
is too rich the machine will normally suffer a “rich bog” in the mid throttle position. To richen the 
circuit you have to raise the needle by lowering the clip.

The needle taper is the angle of the needle at its lower half. This part of the needle fits into 
the main jet. The taper works the transition between the midrange and full throttle/main jet 
(approximately ¾ throttle position). The taper also affects to a small degree, the main jet 
size your carburetor requires. A leaner needle taper will use a richer main jet than a 
comparable engine/carburetor combo with a richer needle taper.

Selecting the proper needle takes some time but the rewards are a stronger, smooth 
power flow and great drivability.

The best way to select the needle is to start with the one that came with your carb. 
Then determine what area of the needle needs to be changed, if any. Place the needle 
clip in the middle position and make sure that the main jet is a size or two larger than 
you think will be the final size. Then make some runs at various mid throttle settings to 
get a feel for what needs to be changed. Some carburetors don’t have a replaceable 
needle jet. On those that do… it is a very effective tuning device, as you can make 
small adjustments in the needle jet to get a centered needle clip.

The main jet is selected in a process similar to the needle. Select a size that you feel 
is a little large and make some WFO high speed runs in 5th or 6th gear and then do 
a good plug chop to see which way, if any you need to adjust. Plug reading is a 
Black Art… that requires some experience in determining the condition of not only 
jetting, but also ignition timing and the overall condition of your engine. This is 
covered in another article.

Run a few laps with varying throttle position and verify that the carb flows smoothly 
from the bottom to the top. If any area needs fine-tuning… make the adjustments. 
This is the seat of the pants method. Now let’s talk about a better method that 
will help you out when the weather conditions change…cause they certainly will!!!!

One of the better carb tuning aids available is EGT monitoring. Exhaust Gas Temperature 
is typically measured in the expansion chamber, at a point about 6 inches from the face 
of the piston. The position of the probe is important in obtaining an accurate reading. 
If you look at the header burn pattern you will typically see a coloration difference in 
the first 3-6 inches of the pipe and you want to be just to the right of the darker area.

EGT is a relative number to your specific engine, and will vary from engine design to 
engine design. You will see lots of numbers used, but you have to find what number 
your engine produces. A higher compression ratio or timing that is advanced will 
tend to produce EGT’s that are lower than the 1200° that a lot of people use…
including myself. Depending upon the engine characteristics your max EGT may 
only be 1100° and to be on the safe side you would need to run an EGT of 1000°. 
Don’t get caught up with the number so much as the method.
To understand how you can use EGT to tune by, you need to be brought up to 
speed on some important factors. One of which is that if you were to look at a 
graph of EGT, you would see a bell curve. This curve would show that the EGT 
rises to a peak and then starts to decline. On the left side of the peak, we are 
richer than stoichiometric, and on the right side of the peak, we are leaner. 

This temperature curve is controlled by the air/fuel ratio and will peak at a 
stoichiometric ratio of 14.7 parts of air to 1 part gasoline.
Stoichiometric who/what??? The combustion of gasoline is a chemical reaction 
that, like any other chemical reaction, has a specific a stoichiometric ratio. 
Remember Chemistry 101, probably not, but you were told that stoichiometric 
when referring to combustion is the ratio of air to fuel at point where the 
combustion reaction is the most efficient.

Now this would all be really cool if we were driving a lean burn Chrysler, 
where we were concerned about fuel economy and emissions, and had 
a sophisticated computer system to keep it spot on … but the last I heard… 
we were trying to tune our engine for the best power…not economy, and 
keep it from melting down. That requires a little richer mixture ratio of about 12.8 
parts air to 1 part fuel, for best power and also leaves us a safety cushion.

During WOT acceleration EGT will rise to the peak and then start to decrease 
after the peak, if we are lean. We need to control where the engine runs by 
controlling the jetting so that we are on the left side of peak, near the richer ratio 
of 12.8 to 1 point for best power. If we jet at the peak, we will be at the mercy of 
a lot of load and weather variables, and without a knock sensor that is tied back 
to a computer to adjust the mixture and timing to fix things for us, we could very 
easily go lean, which is on the right side of the peak, and be in meltdown/detonation 
city… and that isn’t a friendly town to be visiting.

So why don’t we use an air/fuel ratio meter instead of an EGT meter. First of all
 you could, and it would work fine. It’s purely a matter of cost and durability. 
An air/fuel ratio meter with the appropriate wide band O2 sensor is about 3X the 
cost of a simple EGT system and really doesn’t tell us any more than the EGT does. 
Sensors tend to deteriorate due to the higher oil content of a two-stroke exhaust. 
Of course…if you run a multi-cylinder engine then you need a probe and gauge for each pipe.

So… How do we tune using EGT? It’s really pretty easy to do, if we remember 
what we just learned about jetting in general, and how the EGT works.
EGT should rise rapidly with the RPM as you accelerate and should come 
to a point and stay there the time you are at full throttle and a constant load. 
If your EGT rises to a peak and then starts to fall as you maintain full throttle, 
you are now on the right (which is the wrong) side of the curve and lean.
Normal reading will vary slightly from machine to machine, due to slightly different 
probe positions and engine characteristics. So they are relative to a certain degree. 

When you’re at the peak… you’re at the stoichiometric ratio of 14.7 to 1 and EGT
 will typically be in the neighborhood of 1300°F or so. Remember that you are looking
 for a peak…not a specific number, and this number is relative to your engine only. 
We want to run on the upside of the curve at about 12.8 to 1 and that will normally be 
100°F lower than the peak reading.
A typical Keihin main jet will change the EGT close to 50°F per jet size change. 
Using this example…let’s say we had a peak of 1300°F running a 180 main jet. 
To drop the temp by 100°F, I would need a main jet that is 2 sizes larger, and would 
end up using a 185. At this setting your plug and piston reading should show you a 
great burn pattern and color. As you accelerate the EGT should rise quickly to 1200°F 
and stay there. Now that I have the machine jetted spot on… I make note of the weather 
conditions, particularly the Air Density and call my baseline a 185 main jet at whatever 
the AD was. Let’s say the AD was 92% for my baseline… it’s getting close to my first 
heat race so I look at the AD and see that it’s now 89.5. What now coach? I can’t make 
a few laps around the track to see what my EGT is, so I need some additional help to 
be jetted correctly for the upcoming heat race.

AD to the rescue!!! In layman’s terms… air density is a term describing how dense 
the air is at any instant of time. In other words…how much does the air weigh. 
Since the fuel mixture ratio is based on the weight of the air with respect to the 
weight of the fuel, it looks like we might have a valuable factor to help us out with 
our jetting dilemma.

Get yourself a mini weather computer station that gives the AD along with the 
temperature, humidity and barometric pressure and a whole bunch of other stuff. 
The cost is cheaper than the cost of a top end rebuild and will pay for itself many 
times over. If you use a Relative AD gauge, remember that it doesn’t take into 
account the humidity, and since humidity plays a part in the true AD, you need to 
know it to find the true AD or corrected AD.

AD is directly proportional to barometric pressure and is inversely 
proportional to humidity, temperature and altitude. Simply put…if the 
pressure goes up, the AD goes up. With this increase in AD my jetting is 
leaner because I have more air, and I need to install a larger main jet to get 
back to the correct air fuel ratio or EGT. Conversely, if the humidity or temperature 
goes up, the AD goes down. This causes my jetting to go richer, because 
I have less air and I need to install a smaller main jet.

You’ve heard you wife or girl friend say she was having a bad hair day…well you’re 
having a bad air day in racing. Temperature and pressure play the largest role in the 
AD equation.

Ok… now that I have this super duper info…what do I do with it?? Simple…for 
every 2.5% increase in AD, you increase your main jet one size. In our example 
several paragraphs above, our AD dropped 2.5%, so I need to decrease my main 
jet by one size to get me back in the ball game. Make sure to error on the rich side
 if you’re between numbers.

During the warm up lap I’ll take a look at my EGT, and if everything is according 
to Hoyle, it should be right back up to 1200°F or what ever my baseline number 
was…or real close to it. Now you can race with the confidence that you jetting is 
correct and you’ll have the most power you can for the situation. It’s really not a 
complicated as it might seem reading this, and after you do it a few times it will 
become old hat!

Some people like to use a CHT probe along with an EGT probe. This 
method is particularly effective on an air-cooled engine and not as effective 
on a water-cooled engine due to the slower rise time of the water-cooled 
head temperature.
Some rules of thumb…
EGT and CHT rise together… all is cool
EGT start to fall and CHT rises… you are lean and typically a sign that detonation has set in.
EGT and CHT are both low is a sign of too rich.

Now you have your baseline jetting setup and a plan for making intelligent jetting 
decisions. Make a note of all the weather information that the jetting info was based upon.
Does it require some back and forth adjustment to get it right sometimes… yep, but your 
chances of getting it right the first time are much better if you go from bottom to top. It’s 
not unusual for me to make 5-6 jet changes in a day of racing due to changing conditions. 
I’ve found that you can typically make 2-3 main jet changes before you have to move the 
needle, but every situation is different.

I have people come up all the time and ask… “What main jet are you running?” My s
tandard answer over the last 42 years of racing has always been “Hopefully the right one!” 
Every engine is different to a certain degree and every driver/rider is certainly different in 
their style of getting around the track or terrain.

What jet works for me, may or may not be right for you in the same situation. 
That’s why I never recommend a specific jet unless I know the history of the engine/rider, 
and also know what his baseline setup is.

You can find Bill here at Power Sports Tuning