Need to know how to adjust the pilot jet on a dirt bike carb, but are intimidated by this task? Jetting a carburetor is often seen as “mysterious”, but I’m going to make it simple for you.
In this article I’m going to show you what the pilot jet does, why it’s so important to get it properly tuned, and then how to easily adjust it.
What does the pilot jet do?
The pilot jet, also referred to as the slow jet, controls the fuel mixture in a carb from 0-¼ throttle position, roughly. It’s the most important jet circuit for many riders because it affects so many things.
Does the pilot jet affect idle?
Yes, the pilot jet definitely affects the idle. You don’t need to adjust the idle screw until you have properly adjusted the pilot jet circuit and air screw or fuel screw.
The pilot jet affects:
- Starting – cold or hot
- Low-end throttle response – smooth or surging
- Low RPM torque – helps get over obstacles with less clutch work
- Idle speed – keeps the engine running
- Spark plug fouling
- How easy your dirt bike is to ride – most of us spend the most time at 0-¼ throttle
Where is the pilot adjustment screw?
The air/fuel mixture screw is simply the “fine-tuner” for the pilot jet. Once you figure out the proper pilot jet size for your dirt bike carb, then you can adjust the air/fuel screw to get the “best performance” possible with smaller adjustments.
The air screw or fuel screw is located on the side of the carburetor on your dirt bike. There’s an easy way to tell if you have a 2 stroke vs 4 stroke carb.
The air screw on a 2 stroke carb is behind the throttle slide (closer to the air box). The fuel screw on a 4 stroke carb is in front of the throttle slide (closer to the engine).
Bogging at full throttle?
If your dirt bike is bogging at full throttle, then there’s a good chance that it’s not the pilot jet causing this issue. The more likely reason is an incorrect main jet in the case of a jetting problem.
What happens if the pilot jet is too big?
A pilot jet that is too big will generally have some specific symptoms. You may just notice one, or it may have all of these signs.
A pilot jet that’s too big will have these symptoms:
- Be hard to start – harder when the engine is hot
- Sputter/hesitate when trying to accelerate at low RPM/throttle opening
- Foul spark plugs more easily because it’s running too rich (too much fuel in the mixture)
If you notice one or all of these symptoms, then it’s time to try leaning out the pilot jet or air/fuel screw.
How do you lean out a carb?
To lean out the air screw on a 2 stroke, turn the mixture screw out (counterclockwise). To lean out the fuel screw on a 4 stroke, turn the mixture screw in (clockwise).
Both of these are reducing the amount of fuel and increasing the air going through the pilot jet circuit. If this helps but you’re still getting the above symptoms, then it’s time to replace the pilot jet with a smaller (leaner) size).
How do I know if I need a bigger pilot jet?
A pilot jet that is too lean or too small will also have similar symptoms, but your dirt bike will react almost the opposite.
You need a bigger pilot jet if:
- It bogs at low RPM/throttle position
- It’s hard to start when cold
- The engine overheats more easily
- Coolant steams out the overflow hose
Other reasons for lean jetting
If you have symptoms of a lean pilot jet, such as hard or impossible to start when cold, it may not be the pilot jet that is the issue. If your dirt bike has been sitting for a while then it may just be a dirty/clogged jet or passage.
If that’s the case, then a simple but proper carb cleaning may be required.
When should you increase pilot jet size?
When the pilot jet is too small, causing a lean fuel mixture, then it’s time to increase the jet size. If you richen the air screw or fuel screw and it still doesn’t help enough to get rid of the symptoms, then you need to go bigger on the pilot jet.
How do you adjust a pilot jet on a dirt bike carburetor?
Once you’re ready to actually adjust the pilot jet, it’s time to drain the fuel and turn the petcock/fuel valve off. Most dirt bikes have just enough room for you to do it with the carburetor still on the bike.
Loosen the carb and rotate it so the bottom is pointing towards you, then remove the float bowl. There will be some residual gas in it.
The pilot jet is the small brass screw part up in the bottom of the carb body. Simply take a small flat-blade screwdriver and remove it. Use the proper size tool or else you’ll round the flats and it will be hard to install/remove again.
Now just put the new size pilot jet back in and just reverse everything else. That’s how easy it is to adjust the pilot jet on virtually any 2 stroke or 4 stroke dirt bike!
How to prevent a catastrophic failure
Properly jetting your dirt bike is just one way to make it reliable so that you don’t have a major failure or breakdown. I want to show you 3 more simple ways to make your dirt bike reliable that many riders fail to do. Click here to learn more.