Home of K-fab & The Deztaz

Technical Information

Chassis Tuning Wonder what things like camber, caster, toe in/out are? Read and learn.

Head Gear How to get a properly fitting helmet.

Jetting How to Jet your carb.

Shocks Theory on how they work, adjusting and setting them up.

Springs How they are rated, multi rate setups, formulae for figuring out rates.

Tire Information Charts on tire sizes and other useless info.

Carb Jetting and Adjustment

I can't stress how important it is to use GENUINE MIKUNI or KEIHIN jets in your carb. Aftermarket jets just aren't manufactured to the same tolerances and can vary quite a bit from OEM specs. The Mikini or Keihin jets may cost a few cents more, but the phrase "You get what you pay for" really does have truth behind it. A couple extra bucks here is a great way to keep from destroying an engine.

You can find diagrams, part numbers and a lot of information on the majority of Mikuni and Keihin carbs on Sudco's web site.

The entire idea of a carburator is to meter fuel at a certain percentage to the amount of air drawn in by the engine at all rev ranges from idle to wide open.

Most carb have three major circuits (some have more, some have less) to cover the tuning range:
An Idle Circuit.
A Mid-range Circut
A Wide Open Circut

These circuts overlap each other just a bit, so the transition between them is smooth and unnoticed.

Things such as atmospheric pressure, humidity, altitude and temperature all have effects on the tuning of a carb. The more modified an engine is, often - but not always the case - the more sensitive it may be to jetting. You'll find out by tinkering with your individual setup.

One very valuable tool I've used for a long time is an Air Density Gauge. It lets you have a visual idea of just how much air is available for the given surroundings. An example would be when I race in Mexico, (sea level, usually 95+ degrees F) my gauge reads from 100 to 110%, but up in Snowflake (6,000 feet elevation, 75-80 degrees F) the gauge will read 65 - 70% I drop three main jet sizes. Eventually you'll be able to look at the gauge and interpret the jet size needed by the air density reading.

Start keeping a log of your tuning. WRITE IT DOWN in your manual, or a notebook or some place that you'll be able to go back and find. (I write my information on top of the carb caps with a permanent marker as well as in a note book)

Write down the following stuff:
Main Jet Size
What position the clip on the Needle is in (top groove is #1, bottom groove is #5)
Make note of the Air Screw setting too. To do this, screw in your Air Screw until is just quits moving while counting the number of turns it takes to close it. DO NOT TRY TO TIGHTEN IT, just turn it in until it stops and count the number of turns. Do this SLOWLY and pay attention. Keep track in 1/8 increments. Write this number of turns down too. Most of the time this is somewhere between 1-1/2 to 2-1/4 turns.

There's also a jet called a Pilot Jet. It helps with idle and just off idle tuning. You have to take the four phillips head screws out of the bottom of the carb and pull off the float bowl to find it. (To remove the float bowl, make sure you use a screwdriver that fits into the heads of the screws snugly - they call them butter bolts because they'll strip just like they were made of butter - pain in the ass). The Pilot Jet will have a number on it too. Anywhere from 20 to 60, depending on the carb and application.

It is very important that you write stuff down. It gives you a base line to always go back to if you need to.

Since a large part of the carb tuning involves the Main Jet, let's start with the basics and learn how to change a Main Jet. It's very easy to do.

You need a 17mm wrench and a 6mm socket wrench or a screw driver with bits that you can change. Even better is get a Jet Wrench Kit - Dennis Kirk sells a nice one that comes with a small case that will hold a number of jets. The wrench will fit all Mikuni and Keihin hex jets - which are the most popular.

Loosen the clamps that hold the carb in intake and air track boots. Rotate the carb so you can see the large nut on the bottom of the float bowl.*

Use the 17mm wrench to remove the nut (right tight, left loose) and under this nut, you'll see the Main Jet. Take the bit out of the screwdriver, slip the screw driver over the jet and unscrew it (counter clockwise). You now have the Main Jet out. Easy, huh? When you put it back in DO NOT overtighten it. Just snug it - no macho man crap, just snug. A jet's just a small brass piece with a hole through it, so it's very easy to twist the threads right of the body of the jet, which leads to a bad day in the shop (and is usually accompanied by a good amount of swearing).

Some Mikuni carbs (most of the TMX line) have a small brass washer that the Main Jet fits into. It often comes out with the Main Jet - MAKE SURE YOU DO NOT LOOSE THIS WASHER!!! It is extremely important and if you don't put it back in, the jetting will NEVER be correct. You'll chase your tail for hours and get nothing.

On the side or on the bottom of the Main Jet you'll find a very small number inscribed in it. This is the jet size. The larger the number, the larger the jet. The more fuel it will flow and the richer the carb will be.

Okay, let's get down to the tuning aspect of your carb. You'll want to do this each time you replace a part - especially if you put on a new performance part. (new air filter, air filter setup, pipe, etc.)

You should have a little section in the book now that says something like the following:
September 28, 2002
Main Jet - 150
Needle - #3 groove
Pilot Jet - #35
Air Screw - 1-3/4 out
Plug - BR8ES

Let's start with the Main Jet:
The Main Jet only works on the upper end of the throttle - approximately 3/4ths to wide open.
Make sure you have a new spark plug installed.
Take your ride out and get it warmed up.
Run it full throttle, get it going as fast as it will go for a block or two and then hit the kill switch, close the throttle, put it in neutral and roll to a stop.

Pull out the plug.

What does your spark plug look like after a wide open throttle (WFO) run?
What color is the electrode? (the part that has the little tit sticking out of it under the little bar)
Is it black? If so, then it's rich. Drop the Main Jet size ONE
Is it white? Then it's lean. Raise the Main Jet size ONE
It should be about the color of a pancake or waffle.

For now, change Main Jet sizes in ONE STEP increments. (one size is not 150 to 151, it will jump say to a 152 or 153 - same the other way - prolly a 147 or something near that)

Go do a plug chop again. What does it look like now? If it's still white, then you're still lean, if it's still black or dark, then it's still rich. Change the Main Jet accordingly and go do the plug chop again. Keep it up until you end up with a nice brown plug.

Generally speaking, you should be able to use the same spark plug for these runs. If the plug came out black and gooey, you're extremely rich and may have to use a new plug for good readings. Lean out the Main Jet a size or two to get close on the jetting and then put a new plug in and finish out the plug chop runs. Use your best judgment.

Now, let's go look at the bottom end of the carb tuning.

In general, a TWO stroke carb has an Air Screw adjustment - which will be on the AIR FILTER SIDE of the carb's slide.

Most of the time, the air screw is set about 1-1/2 turns out from fully seated - this is a good base line starting point.

At first, adjust the Air Screw in 1/2 turn increments so you can see the difference. Then start being more finite by making 1/4 turn adjustments. You'll eventually get to where you only need to make adjustments in 1/8 turn increments.

Get your ride idling in neutral (warm it up fully first)
Quickly nail the throttle, going from idle to wide open in a smooth manner, but quick manner.

Just stabbing the throttle as fast as you can may cause enough disruption in the flow of the intake that the engine can't keep up - eventually, when the carb's tuned correctly, you should be able to nail it and have the engine react correctly. You'll see how this works as you tune the carb.

What does the engine do?
Does it hesitate and then rev up?
As it comes back down to idle, does it seem to hang at a higher idle for a minute and then settle down to normal idle?

If it does this you're Too Lean on the Air Screw adjusment. You need to richen it up by turning the Air Screw IN 1/2 turn and then repeat the test.

Adjust as necessary, repeating the test until you have a nice response.


Does it sound like it bogs a bit?
As it comes back down to idle, does it seem to drop to a low idle (or even die) and then come back to normal?
If it does this, it's Too Rich. Turn the Air Screw out 1/2 turn.

Adjust as necessary, repeating the test until you have a nice response.

Remember: Adjust the Air Screw in 1/2 turn increments at first then fine tune with 1/4 turn and then 1/8 turn increments.

Repeat the proper adjustment until it responds without hesitation and drops back to idle without hanging or falling and then coming back up. Expect to do the bottom end adjustment at least three or four times before you get it right.

Go write down your findings

Now that you have a base line setting, try adjusting the air screw a full turn either way and see how the engine reacts. You'll soon see what the air screw does. Since you WROTE DOWN what you found, you'll be able to go right back to that adjustment point.

Just a little FYI: Four stroke carbs have a small adjustment screw on the ENGINE side of the carb's slide - it's not an air screw, it's a fuel metering screw, so screwing in the adjustment leans the circuit. NOT what we want on our two strokes.

Mid Range Tuning - The Needle

Now that we've done the top end and low end adjusting, what was the overall tuning direction of what you did?

Did you end up Leaning out the upper end and the bottom end? If so, chances are very good that it's Rich in the mid range too. (and vice versa if you were lean everywhere)

If you Leaned out on top and Richened on bottom (or richened on top and leaned out on bottom), chances are that you won't need to play with the Needle's settings.

This is where the Needle comes into jetting.

The Needle is inside the slide. You have to remove the cap that the throttle cable comes into. It just unscrews, pull it out and a large spring and a slide with the Needle will come out.

The Needle is removed by pulling the spring out of the slide body. You should be able to see it now. T here might be a retaining clip or washer on top of it. (once again, this depends on the carb - go use that manual!)

Now, this is a point that confuses people a lot.

To lean out a Needle, you need to move the Needle DOWN into the emulsion tube. You do this by moving the little c-clip Needle UP.
You richen the Needle by moving it UP - done by moving the clip DOWN.
You'll hear people talking lowering the Needle by raising the clip (leaning) or raising the Needle by lowering the clip (richen). You just have to stop and think about what was said.

Get the vehicle rolling, about 1/4 throttle and then feed in the throttle (you want to take about 1-2 seconds feeding in throttle - quickly, but not just snap it open)

What does it do?
Does it come off the bottom end well and then seem like it gets sluggish?
Then you're too rich - drop the Needle by raising the clip ONE position.

Does it seem like it wants to pull through, but just doesn't have any go? Then it's lean.
Raise the Needle by lowering the clip ONE position.



Depending on how out of range your jetting is, you may have to work with the needle, air screw and main jet a couple times to get it right. Don't be discouraged if it doesn't work the first time - learn from it instead.

Worn Out Parts in your carb?

A little background first: This issue is more prevalent in four stroke machines. It occurs more quickly on them as they have a higher suction pressure than a two smoke does on the intake stroke - but enough cycles on any carb can eventually cause this.

(just a little side note: Most higher performance four strokes (MX Bikes and quite a few street bikes) use some version of the Keihin FCR flat slide carb. Because of the intake pulse's suction power, these carb's slides actually runs on bearings to keep them from getting stuck in the carb body during the intake pulse. That would be very bad! - it's also why so many of the four strokers use a push/pull throttle cable assembly - so you can force the slide closed if something inside the carb's slide sticks or such)

Anyhow, back to the info...

The needle, as it sits in the slide of any carb, is slightly loose - just the way they are by design. Some needles are held in by a small spring, some by a clip, some held by a small arm and a screw. But all of them have the ability to move just slightly.

This movement (especially as the carb gets older and starts to wear) allows the needle to rub against the inner wall of the needle jet (the brass tube that the needle slides down into and that the main jet screws into from the bottom of the carb - sometimes called the emulsion tube).

What happens over time is that the needle will eventually cause the needle jet to oval out, changing the jetting from mid-range on up towards the top end (not as much on top end though, as the main jet limits the maximum amount that the carb will flow when wide open and the needle's not really doing much when it's at wide open either).

So, if you're chasing what seems to be a rich mid-range jetting problem and you have a old carb or a carb with a lot of hours on it, pull the slide out and look down into the needle jet and see if it's starting to oval. You'll see the ovaling on the engine side of the carb. - think egg shaped

Most of the newer carbs, along with the higher performance older carbs, have replaceable needle jets, so if yours is looking worn, just go get a new one!

You can also do more specific tuning with the needle and needle jet too - there are often options of needle tapers and needle jet diameters. It can get really deep if you're one of those racers that chases that last spec of hp.

Want more, eh?

Here's a great article (that I stole from dhjunkie) on what the parts of the carb are and what they do:

This is from R&D Aerosports page and thought it might help those about what part does what in the carb. (Keep in mind, this was written about a particular carb - the sizes listed are not the only available - see local dealer for more)

What Jet Does What?

The Pilot System: The pilot or idler jet controls the air/fuel mixture mainly from closed to 1/4 throttle. Idler jet comes in sizes 35 to 60 at a 5 unit increments.

The Throttle Valve: The round, flat, or oval cylinder (slide) that rides inside the main carb body is the throttle valve. This part is rarely changed in tuning and the same style is used on all Rotax engines. These valves do come in different cut-away configurations which changes the angle of the diagonally cut surface, but they are expensive to change and hard to find. If you need a leaner mixture you can file down the first taper, which increases the angle of the cut-away. This will lean out the mixture.

The Needle Jet Circuit: This circuit is actually two toning pieces working in conjunction with each other.
The Needle Jet is the cylindrical brass passage located directly above the main jet. This part is available in sizes 268, 270, 272, 274, 276, 278, and 280. The smaller the number, the smaller the diameter of the inner passage and the leaner the condition. The jet needle is the pin that rides inside the throttle valve and out the bottom of this slide into the needle jet passage.
The Jet Needle: This part is available in 6H2, 802, 8G2, 8L2, 15K2, 11G2, 11K2, 15K2, and 15E5U. There may be a lot more available, but it's a well-kept secret, at least in this sport. A general rule of thumb to apply is, needles with a "High Number Code" produce richer mixtures above half-throttle. Example: 8L2 instead of 6L2. Needles with a "High Letter Code" produce richer mixtures below half-throttle. Example: 6P2 instead of 6D2. My recommendation is if you have a mid-range throttle problem, change the needle jet because this part will consistently affect the entire range from 1/4 to 3/4. Or you may want to experiment with your jet needle position.

The Jet Needle Position: This position is determined by which location the holding plate is installed on the jet needle. This position affects the timing of the jet needle versus the throttle side and the needle jet. This relationship requires some thinking. It may require you to read this section several times, but once you understand what's going on, you can make educated changes rather than random experiments.
Moving the holding clip to the top notch (position #1) will delay the timing between the jet needle and needle jet clearance versus the air intake supplied by the opening of the throttle valve. This will produce a leaner 1/4 to 3/4 throttle opening, because there will be more air passing into the engine than fuel allowed. Moving the holding clip to position #4 will accelerate the timing between the jet needle and the jet clearance versus the opening of the throttle valve. This will produce a richer mixture from 1/4 to 3/4-throttle opening. Don't expect miracles! While this change will make a difference, it may not cure your problem.

A common complaint is that an engine will not operate at a given rpm. The engine will either drop or gain rpm without a throttle change. This gap can be as much as 1,000 rpm's and can often occur in the cruise range. Very annoying! A lot of people attach the holding plate position and then, in frustration, go after the needle jet/jet needle circuit. This condition, more often than not, is a result of the tuned exhaust system not wanting to hold that rpm. There is a chance that a mid-range modification may take care of this problem, but in many cases, you are stuck with it.

The Main Jet: This part is easy to understand. It is simply a hole of an exacting size drilled in a hex-sided piece of brass. This passage controls all the fuel leaving the float bowl with the exception of the bypass circuits discussed previously. The main jet is available from 125 to 220 in 5-unit increments, with a few mid-sizes available, such as 146, 158, and 162. The smaller the number, the smaller the opening, and the leaner the condition. This part controls the fuel mixture mainly from 3/4 to full range. The majority of engine seizures and meltdowns occur in this 3/4 to full range. Leaning or richening the main jet will have a small effect on mid-range jetting.

EGT: CAUTION! Most EGT gauges read slow, it is possible to seize an engine with exhaust temperatures as low as 800 degrees. It is advised to start with a rich mixture and only jet down if the exhaust temperatures are low after an extended period of time. We recommend 1050 degrees for climb out and not over 1150 degrees at cruise.

* - This isn't easy to do on a Pilot that hasn't had the radiator relocated. You can't spin the carb in the boots. You have to become a yoga master and work your hand and tools up under the carb from the clutch side. It may actually be easier to remove the air box and intake tract in some cases.

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