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How To Properly Set The Idle Speed On Dirt Bike Carb

Need to know how to set the idle on your dirt bike? Whether you have a 2 stroke or 4 stroke, you’re in the right place!

In this article I’ll show you what affects the idle speed, where to adjust it, how to easily adjust it, and practical tips for making your dirt bike last longer and running better!

Are dirt bikes supposed to idle?

Yes, virtually all modern dirt bikes are supposed to be able to idle. With that said, there are some conditions and things to consider. 

First of all, when you start an engine that’s cold (same temp as the air temp), it most likely won’t idle with the choke or well. Once it’s up to full operating temp it should stay running. 

XR600 Smoking
Gettin’ her warmed up for the day!…

What happens if you let your dirt bike idle too long?

Although you should be able to let your dirt bike run without having to give it some throttle to stay running, you shouldn’t let it idle too long. 

Once the engine is warm, it will continue to get hotter if the engine is running and you’re not riding it. All dirt bikes need to move in order to keep them cool, whether it’s an air-cooled or liquid-cooled motorcycle

Riding on a slower dirt bike is safer
Riding an air-cooled XR200 in the mountains

If you let it idle for too long, it will start overheating. A liquid-cooled dirt bike will start leaking coolant out the overflow hose. 

It can also cause the head pipe to turn red hot.

Don’t idle in gear

You also don’t want to leave your engine running while it’s in gear with the clutch in for too long. This heats up the clutch quicker. 

For example:

You come to a stop at the end of a trail and it’s still in first gear. You have the clutch pulled in while waiting or talking to your buddy. 

If you’re going to stop for more than 10 seconds, either put your bike into neutral or shut the engine off to keep it as cool as possible. 

What should the idle be set at?

Every dirt bike is a little different, but the idle should be set so that the engine can stay running without the RPM being too high. This is generally going to be 1000-1500 RPM. 

Since most dirt bikes don’t have a tachometer to visually see the engine RPM, you just have to go be sound and feel. You can set the idle as low as possible and if it shuts off after a couple seconds then turn it up slightly. 

Where is the idle screw located?

The idle screw is located on the side of the carburetor on most dirt bikes. It’s generally a small screw that can be turned with a phillips-head or flat-head screwdriver. Some idle screws are big enough to turn with your fingers.

The idle speed screw on a dirt bike carburetor is usually on the left side near the middle
This is where the idle screw is located on a Keihin 2 stroke carb

How to set the idle speed on your dirt bike

You simply turn the idle screw until it settles at the speed you want it to. Do this when the engine is fully warmed up and the choke is off.

This is because a cold engine will have a lower idle speed due to the leaner air/fuel mixture. Once the engine is warm, the fuel mixture will stabilize and be consistent throughout your ride.

Pro Tip:
If the weather drastically changes or you ride up or down in elevation, you might need to adjust the idle. As the air changes, the jetting changes along with it, and this affects the idle speed.

Which way do I turn the idle screw to idle?

To turn the idle speed up, turn the screw in, which is a clockwise rotation. To set the idle speed lower, turn the screw out, which is a counterclockwise rotation.

2 stroke dirt bike idle RPM

A 2 stroke dirt bike should be able to idle if it’s properly jetted. The idle speed will likely be around 1200-1500 RPM, but you just have to set the screw to where it keeps your bike running.

Whether you have a Keihin carb or a Mikuni carb, the idle speed is still adjusted the same way.

4 stroke dirt bike idle RPM

Depending on what type of 4 stroke dirt bike you have, the idle speed may be lower or higher.
For example, a lower performance trail bike can typically have a lower idle speed than a high performance race bike. This is because the trail bike is tuned for riding at a lower RPM.

How to adjust idle speed on a fuel injected dirt bike

It’s the same process as a carbureted dirt bike – wait until your engine is fully warmed up before adjusting the idle speed. A fuel injected dirt bike 

How do you fix a high idle on a dirt bike

First, try adjusting the idle speed lower via the idle screw by turning it out (counterclockwise).

Why is my bike revving so high when I start it?

If your dirt bike revs high on start-up then there’s most likely an air leak. Too much air causes it to run leaner. This makes the carb think the throttle is partially open and it sucks in some more gas, which is why the engine revs up.

To fix an air leak, carefully inspect the intake manifold and air boot. It’s often a crack or tear that’s letting air through. It could also be a worn or torn intake gasket on the cylinder/head, or the carb clamp isn’t properly tightened.

If the air boot or intake manifold is torn, you’re going to have to replace it. It’s usually $50 or less depending on your dirt bike.

It’s not safe to keep riding with this problem because it can start revving and accelerating on you when you don’t want it to. This will eventually cause an accident.

Take the time to get this problem fixed or you’re going to pay for it in the long run – trust me, I would know!

What causes hanging idle on a dirt bike?

A hanging idle could be caused by an air leak as well, but there’s a good chance that it’s due to lean jetting. Whether it’s a lean pilot jet circuit, the needle clip is set too lean, or the float level is incorrect.

If you have a 2 stroke, the air screw should be adjusted first, and then the pilot jet if it’s still too lean. On a 4 stroke, the fuel screw is the first thing to adjust.

If none of these fix it, then you can try adjusting the needle clip position. Lower the clip one position to richen it.

A float that’s set too lean will cause all of the jetting to be lean. Re-set the float height and see if that fixes your hanging idle problem.

What causes a dirt bike not to idle?

A dirt bike that won’t idle or stay running is most often caused by these three things:

  • Idle screw is not properly set
  • Air screw/fuel screw is set too rich
  • Pilot jet is dirty or too big

How to fix a dirt bike that won’t stay running

Going off of the above causes, start with adjusting the idle speed screw. If it still won’t stay running, try adjusting the air/fuel screw.

Still won’t stay running? There’s a good chance the pilot jet is dirty/clogged from sitting too long or it’s too big (rich). A proper cleaning of the carb will fix this.

What to do when the idle screw has no effect

Similar to fixing your bike when it won’t stay running, if your idle screw has no effect then it’s probably due to poor jetting. Start with the air screw/fuel screw. If that’s not enough then try going one size bigger/smaller on the pilot jet and see if it runs better.

If none of those work, then your engine might be low on compression. An engine with low compression can’t properly burn all of the fuel, so it runs rich and lacks power.

Whether you have a 2 stroke engine or 4 stroke engine, you may need to do a top-end rebuild if the compression is too low.