Secrets To Jetting A 2 Stroke Dirt Bike [Guide]

What is jetting and why should I learn about it? Even if you know how important it is, maybe you’ve neglected it because it’s intimidating…

Pulling apart a carburetor and changing these tiny jets to change how the bike runs, but where do you start?

Changing the jets is actually not that hard, especially on a 2 stroke dirt bike carb. The harder part is the tuning, but that’s why you’re here. This is the ultimate guide to simplify and understand how jetting works

Why Is Jetting Important?

A correctly jetted carburetor makes a tremendous difference in the torque, mid-range pull, top-end pull, and over-rev of your engine. If you have never jetted your bike correctly, you will almost certainly gain some performance at some point in the bike’s power-band.

What Does Jetting A Dirt Bike Mean?

Jetting a dirt bike is just changing the size of jets and adjusting the needle to change the air to fuel ratio (AFR). A correct AFR will yield the most power, efficiently, reliability, and run the cleanest.

What Can Jetting Do For You?

A cleanly jetted pilot circuit can be the difference between having to clutch the bike out of a turn or not. Hard starting when hot or cold, poor response when opening the throttle, reluctance to idle, all of these are symptoms of an improperly sized pilot jet or incorrectly adjusted air screw.

The needle can make all the difference in the world for the power of the machine in most situations, as it controls the throttle range that most riders spend most of their time using.

A correctly sized main jet could mean the difference between being able to rev out high enough to not have to shift one more time at the end of the straight, or the power falling flat on top and requiring you to make that extra shift.

Dirt Bike Jetting 101

Proper Jetting Cures Many Problems: What’s Your Problem?

Are you fouling plugs? Many people will tell you all sorts of band-aid fixes, from running less oil, to running a hotter plug. Both are incorrect fixes for plug fouling. It’s all in the jetting.

An engine that is jetted too rich will have combustion temperatures that are too low to burn the fuel and oil effectively, leading to deposits and wet fouling of the plugs.

Do You Have Spooge?

There are the rare instances where a mechanical issue, such as a leaking wet-side crank seal, can cause spooge. But, by and large, this isn’t the case. In most instances, spooge is caused by rich jetting. It has nothing to do with how much oil you mix in the gas, or how hard you ride.

An engine that is jetted too rich will have combustion temperatures that are too low to burn the fuel and oil effectively, resulting in deposits, plug fouling, and spooge. Spooge is nothing more than unburned fuel and oil entering the exhaust.

motocross bike mods jetting
Rich Jetting Causes Spooge

What Jetting Changes Do I Need?

The only way to know what jetting changes you will need is by trial-and-error. No one can give you the perfect jetting specs because every bike is different. Every rider has a different style, and jetting is totally weather dependent.

Unless the person telling you what jets to use is riding an identical bike, on the exact same track, at the same time, his recommendations are meaningless. Someone with a good understanding of jetting can get you in the ball park, but you need to do the testing to determine the correct jetting yourself if you want it right.

Jetting is fairly simple, and is a useful skill to learn if you ride a two-stroke and want it to perform at it’s best.

What Does The Pilot Jet Do?

It’s very important that you start with the pilot circuit, because the pilot circuit affects the entire throttle range.

When you are at full throttle, the main jet is the primary fuel metering device, but the pilot is still delivering fuel as well, adding to the total amount of fuel that your engine is receiving.

Dirt Bike jetting chart for tuning each jet circuit.
Jet tuning chart for each circuit.

Before You Start To Jet-Testing Your Dirt Bike

You need a clean air filter, a fresh spark plug (actually you need several plugs to do plug-chop tests for the main jet), and fresh fuel. Set the float level to the proper specs. An incorrect float height will affect your jetting all across the throttle range.

One important detail: Make sure the engine is in good mechanical condition. If your engine has a worn top-end, fix it first. Trying to jet a worn out engine is a waste of time. The same goes for reeds that don’t seal properly, and a silencer that needs re-packing. Worn reeds will mimic rich jetting, and worn rings will mimic lean jetting.

All jet testing must be done with the engine at full operating temperature.

How To Test & Adjust The Pilot Jet Circuit

As already stated, start with the pilot circuit. Turn the air screw all the way in, then turn it out 1.5 turns to start. Start the engine, and turn the idle screw in until you get a slightly fast idle, or hold the throttle just barely cracked, to keep the engine idling.

Turn the air screw slowly in, and then out, until you find the point where the idle is fastest. Stop there. Do not open the screw any farther, or your throttle response will be flat and mushy, and the bike may even bog. This is only the starting point, we will still have to tune the air screw for the best response.

Do I Have The Right Size Pilot Jet?

Now is the time to determine if you have the correct pilot installed in your carb. The air screw position determines this for you, making it very simple. If your air screw is less than 1 turn from closed, you need a larger pilot jet. If it is more than 2.5 turns from closed, you need a smaller pilot jet.

If your engine doesn’t respond to air screw changes, then you either have a dirty carb, or the pilot jet is way too rich. When the pilot jet is way too rich, you are forced to use the idle screw to open the slide so far in order to keep the engine running that the pilot circuit is partially bypassed, and the engine is actually starting to draw fuel through the needle jet.

How To Achieve The Best Throttle Response

Once you have determined (and installed it if it’s necessary to change it) the correct pilot jet size, and tuned the air screw for the fastest idle, it’s time to tune the air screw for the best throttle response.

Again, make sure the bike is at full operating temperature. Set the idle back down (the bike should still idle, despite what you read in the Moto Tabloids), and ride the bike using closed-to-1/4 throttle transitions. Turn the air screw slightly in either direction until you find the point that gives you the best response when cracking the throttle open. Most bikes are sensitive to changes as small as 1/8 of a turn.

The air screw is not a set-it-and-leave-it adjustment. You have to constantly re-adjust the air screw to compensate for changing outdoor temps, humidity and changing elevation. An air screw setting that is perfect in the cool morning air will likely be too rich in the heat of the mid-day.

What Does The Needle Do?

Now it’s time to work on the needle. Mark the throttle grip at 1/4 and 3/4 openings. Ride the bike between these two marks. If the bike bogs for a second before responding to throttle, lower the clip (raising the needle) a notch at a time until the engine picks up smoothly. If the bike sputters or sounds rough when giving it throttle, raise the clip (lowering the needle) until it runs cleanly.

There isn’t really any way to test the needle other than by feel, but it’s usually quite obvious when it’s right or wrong. A too-rich needle can often be felt simply when revving the bike on the stand. The bike will sound rough and raspy when blipping the throttle on the stand. A correctly jetted bike should rev cleanly and crisply.

Gain Horsepower With The Right Main Jet

Last is the main jet. The main jet affects from 1/2 to full throttle. The easiest way to test it is to do a throttle-chop test. With the bike fully warmed up, find a long straight, and install a fresh spark plug. Start the engine, and do a full-throttle run down the straight, through all gears. As soon as the bike tops out, pull the clutch in, and kill the engine, coasting to a stop.

Remove the spark plug, and look deep down inside the threads, at the base of the insulator. If it is white or gray, the main is too lean. If it is dark brown or black, the main is too rich. The correct color is a medium-dark mocha brown or tan.

Main Jet is too rich and causes a black fouled spark plug.
Main Jet is too rich and causes a black fouled spark plug.

Please note that, when reading plugs, the tip of the insulator, threads, etc. are meaningless for jetting purposes. They can tell you a lot of things, but jetting isn’t one of them. Only the mixture ring at the very base of the insulator, inside the threads, can tell you anything about the jetting.

Considering The Slide

The slide is also a tuning variable for jetting, affecting the throttle range from 1/8 throttle to approximately 1/3 throttle. If you can’t clean up the small-throttle jetting on your bike no matter how lean you go with the pilot or the needle, the slide is the next step. With that said, very few bikes need leaner slides.

Jetting A 2-Stroke vs A 4-Stroke

Keep in mind, even though this article is intended primarily for two-strokes, four-strokes also need proper jetting to perform right, although they are not quite as fussy as their oil-burning cousins. The only real difference in the two is with the pilot circuit.

Air Screw vs Fuel Screw – What’s The Difference?

2-strokes have an air screw that you screw in to make the jetting richer, and screw out to make the jetting leaner. 4-strokes, on the other hand, have a fuel adjustment screw that you screw in to make the jetting leaner, and out to make it richer. Their purpose is the same, they just do it in different ways.

Better Power & Reliability

Want your 2 stroke to start easy and make better power, all while being more reliable? Click here to learn more.

-Thanks to Chokey over at ThumperTalk.

Kelley Fager

Kelley started riding a Honda 50 at the age of 6 years old. The passion for dirt bikes started there and grown into a lifelong pastime of riding and learning how they work. Motocross Hideout is the result of sharing his past, present, and future knowledge and experience.

7 Responses

  1. Jack Czerwenka says:

    Can someone please make a video of how to jet and send a link of it to my e-mail (jack.czerwenka@gmail.com) thank you if someone does 🙂

    • Tom Stark says:

      This guide tells you pretty much everything you need to know. Swapping the jets isn’t too difficult, especially if you have a manual. Once the carb is off you can take the float bowl out and the jets should be there.

  2. ram says:

    i want to knw the things used to jet ,that is the things inside the carp by identifing by its picture it is more easier to understand…can any one say me how to jet a yamaha fazer 150c……..pls send me a mail …thanking uu..

  3. denis says:

    ive got a ’93 rm125, been rejetting the carb after having it ultrasonically cleaned but having problems. It has a bit of flat spot between half and full throttle. I went with the fmf’s recommendations on jet sizes since it has a fmf gold series exhaust and vforce 3 reed valves but noticed the flat spot (main jet was a 330) so ive bought a 340 and a 360 main to try out, I went straight for the 360 and a bit stumped now as the flat spot is still there. Ive been adjusting the two screws on the carb but just cant work out wtf is happening. what could be the problem??

    • Tom Stark says:

      Was the float cleaned? What does the plug look like? If changing the main jet isn’t working, I suggest you do some testing with the needle. Go a position or two higher with the needle clip.

  4. Andre k says:

    Hi

    Looks like you guys know what ur talking about.

    My kx 250 86′ started 2nd kick. Then started getting harder to startt
    Gotten to a point were i csnt get it started atcall now. Wet plug with oily residuee
    On it. Put in new plug.nothing.clean the carb.nothing.clean the air filter.nothing…
    Not sure what to do next? Any help? Seems like the jetting could be a n option?
    Was working fine…dont understand how it can go pearshaped if all wss well?

    Andre

  5. Steve Matthews says:

    Awesome article…well done. Truly the best explanation ever.

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