The Ultimate Jetting Guide for Dirtbikes

Even though there are really a dozen parameters you can change in a carby, there are really only two basic circuits in a carby you should know about for tuning most bikes.

1) The Pilot Jet – squirts in fuel at and around idle
2) The Main Jet – squirts in fuel when you turn the throttle

The amount of fuel these put in can be regulated by simple adjustments.

There is a mixture screw that can be turned in or out on nearly all bikes to regulate this.

Most 4 strokes it is called a fuel screw and the more you turn it out the richer it gets. This is usually found towards the engine side of the carb at the bottom.

Most 2 strokes it is called an air screw and the further you turn it out the leaner it gets. This is usually found on the side of the carb towards the air box

DO NOT CONFUSE THESE WITH THE IDLE SCREW – usually a black plastic knob that just increases the revs (not the air/fuel mixture ratio).

This jet is regulated by the needle unblocking the fuel flowing through it. The mroe you turn the throttle the more fuel flows. You can lower the clip on the top of the needle to start the needle further out of the jets way making the fuel flow richer.

Altitude and Temperature

These two things effect your jetting and it is worth considering changing your jetting at least twice a year to get the best power out of your bike (start of summer and winter).

As you go higher in altitude the air gets thinner, which means your bike runs richer, so often you have to compensate by leaning your bike out when riding in the high country.

As the temperature goes up the air also gets thinner and your bike runs richer, so your fuel needs to be leaned off also.

For a starting point for this adjustment, try leaning your pilot by turning your mixture screw (air screw for 2T and fuel screw for 4T – see above post) about 1/4 to 1/2 to see if the bottom end response improves. This can be done on the trail, but just remember where you are at with your settings.

If you get real keen and think the mid is too rich try raising the clip on your needle one position to lean it out. I will discuss plug color to check how accurate your jetting is.

Lucky Victoria
Now if you live in Victoria you are lucky because although we have some good high country the higher we go the colder it generally gets which means the air density does not change drastically and only small adjustments are often needed.

Plug color

When I bike is jetted correctly the porcelain around the electrode will be a nice tan-brown color. White means too lean, black means too rich.

You may of heard the term plug chop. This simply means you ride the bike in 2nd gear flat out for about 2klms and then hit the kill switch, while keeping the throttle open. When you pull the plug the color will indicate if the main jet is correct. You can repeat this procedure at half throttle to give you a bit of a guide about the needle, but it is usually easier to just try a setting and ride it to see if the bike gains response and power.

– iridium plugs are hard too read as they usually have a lot of black carbon around the thread end and porcelain no matter how it is jetted. You have to look at the electrode itself closely.


The fuel screw can either richen up or lean out your bike at idle and just above. This setting has a massive performance influence on responsiveness off the bottom and good starting!

There are a couple of easy ways to get your head around adjusting this.

After giving the bike a quick rev to about 1/2 throttle
– If the engine hovers just above idle for a second or two then settles the bike is too lean. Wind the fuel screw out a 1/4 of a turn or more.
– If the engine drops below idle for 1/2 a second and then returns to idle it is too rich. Wind the fuel screw in half a turn to lean it out.

If the bike starts easily on choke on a average temp morning and then can be turned off after 30 seconds the mixture screw is usually pretty well set.
– If the bike needs to stay on idle for a lot longer it is usually too lean (turn the fuel screw out to richen it up)
– If the bike starts without choke it is usually too rich (turn the fuel screw in to lean it off)
If it is a very hot or cold morning then this should not apply. Cold morning will need a lot of choke and hot mornings will need very little.


Two strokes have a brass AIR SCREW adjuster on the left side of the carb, towards the airbox low on the carb.

If you turn this outward (anti-clockwise) it will lean up your idle and lower throttle positions.

If you turn it inwards it will richen up your idle and lower throttle positions.

– If your bike feels slow revving down low or splutters and won’t rev clean off the bottom, chances are the air screw is set too rich. Wind it out 1/4 of a turn at a time until it revs clean.

– If the bike idle hovers or you can not get it too idle even when you wind the idle screw (usually a black knob in the center of the carb on the left side) all the way in, you could be too lean. Wind the air screw inwards.

AIR SCREW SETTING IS USUALLY ABOUT 1 1/2 to 2 1/2 turns out.

If you want better idle and/or low down (throttle position) response adjust the fuel/air screw. Only change the pilot jet if you are winding this screw below 1 turn or over 3 turns.

If you want more mid range or top end (throttle position) response, then look at the main jet/needle clip circuit. Unless the plug color is white or black after a good wide open throttle ride, leave the main jet alone. Adjust the clip on the needle (higher to lean the jetting out, lower to richen the jetting up).


If you have dropped a bike and it will not start often it is flooded with fuel.

The best way to clear this fuel is to open the throttle fully. Do not pump the throttle, just hold it wide open. This lifts the slide and lets more air in, leaning out the jetting.

Then give the bike several kicks or cranks on the button. I probably wont start (2 strokes may start, get ready to let go the throttle) but it will clear the fuel.

Then let go the throttle and start normally!

Only vapor ignites, not liquid petrol. When the motor is hot it evaporates quickly, but first thing in the morning it is cold and the fuel condenses on the head and barrel. So by pulling out the choke, it richens the mixture up to compensate for all the fuel that stays as a liquid.

When your bike starts the motor heats up and makes the fuel vaporize quicker. Often it will blow a bit of smoke, this is the unburnt fuel mixed.

Like with a flooded bike, when you turn the throttle you lift the slide up and actually lean the air/fuel ratio out. If you bike is not warm enough and vaporizes the fuel correctly it will stall. You may need to richen the fuel/air screw up a bit during winter to help starting.

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