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Dirt Bike Won’t Stay Running: How Easily To Make It Idle

Dirt bike won’t idle without throttle after you got it started? Whether you have a 2 stroke or 4 stroke dirt bike that won’t stay running, you’re in the right place!

In this article, I’ll show you every common problem that causes a dirt bike or motorcycle to not idle, how to fix it, as well as the less likely issues in case you’ve already tried everything else.

À 2 stroke or 4 stroke dirt bike that won’t stay running is actually a fairly common problem. Some causes are very easy and quick to fix, but some bikes require a little more investigating to solve the real issue.

Dirt bike won't idle fixes

2 Common mistakes when troubleshooting your dirt bike problem

Before you get started, you need to know a couple of things so that you don’t break your dirt bike or motorcycle, and that you actually understand what you’re doing to make it much quicker the next time you have a problem.

Beginner mistake #1

The first tip is to only do one thing at a time – one of the most common beginner mistakes because you’re in a rush! If you’re messing with multiple things, you’re a lot more likely to cause even more problems.
If you did 2 or 3 different “fixes”, then you may miss out on a “learning experience” because you don’t know which one thing actually solved the problem. 

Dirt Bike Wont Start Thumbnail Dirt Bike Won’t Stay Running: How Easily To Make It Idle
WHY didn’t you make one change at a time??

Beginner mistake #2

The second tip is to always start with the easiest thing, then work your way to more involved ways to identify the root problem.
Why would you start with the most difficult and time consuming option? Save your time and money and pick the easiest thing to check first. For example, is there enough gas in the tank, and is the idle screw properly set?

Dirt bike won’t start?

Most of these fixes are for a dirt bike that starts but won’t stay running – if your dirt bike won’t start at all, then you need to read this article first on getting it to start.

Dirt bike starts then dies?

If your dirt bike starts then shuts off, whether it’s a 2 cycle or a 4 stroke engine, then it’s likely not getting enough air or fuel.
When it doesn’t get enough air, the fuel mixture is too rich for the spark plug to ignite it. This could cause the plug to foul if your 2 stroke dirt bike starts then dies too many times.

If it’s not getting enough fuel then the air fuel mixture is too lean. Squirting starter fluid in the engine or using the accelerator pump to spray some gas into the engine can help get it started, but the jetting may be too lean to keep it running.

Dirt Bike Compression Test Dirt Bike Won’t Stay Running: How Easily To Make It Idle
Kick Starting An Old Honda XR200

Dirt bike won’t idle without throttle

Maybe your dirt bike starts and runs but it just won’t stay running unless you keep giving it gas. This could be caused by a low idle, a rich fuel mixture, or a worn out engine.
Keep reading to learn about each problem and how to fix it.

Are 2 stroke dirt bikes supposed to idle?

The simple answer is “Yes”. All dirt bikes, both 2 and 4 stroke are made to be able to idle for a reasonable amount of time. 2 strokes may have a more erratic idle just because of the way the engine works and the timing of each power stroke. 

A 2 stroke that doesn’t idle for more than 30 more than likely has the idle set too low or the jetting needs to be corrected.

The YZ125 is a good choice if you want to start racing motocross
My ’06 YZ125 could idle for a while when jetted properly

2 stroke won’t idle

Whether you have a 2 stroke or a 4 stroke, if the idle adjustment is set too low, the engine won’t stay running. 

Turn the idle screw in and see if your dirt bike stays running. You may have to turn the idle down slightly as the engine temperature warms up. If that doesn’t fix it, then it might need the air/fuel screw adjusted, which I’ll cover shortly.

Idle screw location on a Keihin PWK Air Striker 2 stroke carburetor
Idle screw on a Keihin 2 stroke carb

Dirt bike won’t stay running when cold

Some dirt bikes, especially older models, are more cold blooded. This means that it takes longer for the engine to fully warm up to operating temperature.

New fuel injection dirt bikes will automatically compensate and richen the fuel mixture to keep the bike idling, but carbureted bikes require a little more manual effort. 

Starting a dirt bike when cold

A cold engine needs a higher fuel/air mixture in order to run because it’s harder to burn efficiently at lower internal temps. As there engine temps rise, less fuel is required to keep it running.

This is why some dirt bikes need to have the choke on left longer to keep them running without needing to give it more gas by twisting the throttle. If your dirt bike keeps shutting off when you turn the choke off, then it might not be fully warmed up.

YZ250 Cold Riding In Snow Dirt Bike Won’t Stay Running: How Easily To Make It Idle
A cold engine simply means it’s at “air temperature” – not to be confused with cold weather

Fuel/Air screw needs adjustment

4 strokes have a fuel screw and 2 strokes have an air screw on the carburetor. They both perform the same type of function, but in opposite ways.

The purpose of the fuel/air screw is to fine tune the adjustment of the pilot jet circuit. This circuit most affects starting your bike as well as throttle response and power from 0-¼ throttle inputs. 

Properly jetting a carb is important if you want the best throttle response, power, and fuel efficiency possible. Most jet tuning is fairly easy if you just take your time and change one thing at a time.

Fuel Screw On XR250 Carb Dirt Bike Won’t Stay Running: How Easily To Make It Idle
The fuel screw location on an old trail bike carburetor

Pilot jet is wrong size

This is the next step if the fuel/air screw adjustment didn’t fix the idling problem. A pilot jet that is too big gives too rich of a mixture that there engine can’t fully burn.

You may be able to get your bike started, but it will have a hard time staying running. Going leaner/smaller on the pilot jet may cure the problem and get your bike to idle properly again.

Pilot jet circuit is dirty

A dirty carb, and more specifically, a dirty pilot jet circuit is one of the most common reasons why a dirt bike won’t stay running.
Other common signs of a dirty carb:

  • Hard to start
  • Bogs or runs rough
  • Only runs on choke
  • Backfires
  • Low/or hanging idle
  • Lack of power

A carburetor gets dirty because it sits for a period of time. If fuel is left sitting in the carb then it will deteriorate and start to gum up after a while.

Dirt bike won't start because of dirty carb and jets
Dirty gas clogs up the carb jets and circuits

This is especially true with cheap regular pump gas with high amounts of ethanol. Running a high quality gas, such as a non-oxygenated had with no ethanol, will last much longer before breaking down and causing problems in your gas tank or carb. 

EFI Injector Is Dirty

A dirty or clogged fuel injector can also cause engine running problems. If your fuel injected dirt bike won’t idle or stay running well after sitting for several months, try running some fuel injector cleaner through it.

Sometimes just running an engine for a little bit will clean out the junk that was sticking in the fuel system. Otherwise, running a fuel injector cleaner (Amazon) may be necessary to get it running smoothly again.

How clean is your air filter?

An air filter that isn’t regularly maintained will build up dirt and other foreign materials on it over time. The filer is there to prevent these objects from entering the engine. 

Dirty Air Filter causes spark plug to foul
Dirty Air Filter After Riding In Sand

But if the filter is so dirty then it won’t allow enough air through the carb/throttle body. This creates a rich air/fuel mixture, similar to what a pilot jet that’s too large causes.

If your air filter hasn’t been cleaned or replaced within your last few rides, it needs to have a fresh air filter installed.

Fuel Filter Clogged/Line Kinked

Adding a fuel filter inline with the gas line can be a great way to help prevent any sediment or foreign materials from entering the carb or fuel injector.

However, if the filter has collected so much sediment and junk in it, not enough gas will flow through to the fuel system, making the engine run rough.

What to do when fuel filter is dirty

If you can see the filter is dirty, replace it and test your results. Also check for a Kinked fuel line.

The line from the gas tank to the carb/fuel system will rarely be straight, but it should never be kinked or or pinched. This will also not allow enough gas to reach the fuel system.

Petcock deteriorated from bad old gas in dirt bike
What happens when you leave pump gas in the tank for years.

Gas leaking out the overflow

Proper float level is critical to the jetting. If the float is set too low for then the engine will starve for fuel. A float level that’s too high will often flood over and gas will spill out the overflow tube.

If your float level (also known as float height) is not correct, your dirt bike will run poorly. 

How to properly set your carb float height

Check your OEM service manual for the float level specs. It’s not very difficult to adjust the level.

You just need to remove the float bowl, either by rotating the carb or removing it from the intake boot to get access to the float bowl. 

Once the float bowl is off, remove the float pin and float, then carefully adjust the metal “tang” in the proper direction to get the correct float height. Re-install everything and see if that solves your problem.

Carb Float Height Dirt Bike Won’t Stay Running: How Easily To Make It Idle
This float looks to be slightly high and should be adjusted.

Air Leak

A pesky air leak will most certainly cause problems. Not only can your engine run poorly, but a leak will allow dirt and other materials to more easily enter the engine.

This will greatly reduce the engine life. Dirt is an engine’s worst enemy, which is why it’s so important to keep your air filter clean if you want an engine to last as long as possible.

How to easily test for an air leak that can’t be seen

If you don’t visually see any gaps or cracks in the intake boot or box, there’s an easy way to test for air leaks with the engine running. Spray a little bit of a flammable substance (such as carb/brake cleaner) around the intake boot.

If you notice the RPM drop or rise, then you most likely have an air leak. A part or bolt is either too loose, cracked, or a gasket needs to be replaced.

troubleshooting dirt bike carb leaking gas
The air box and intake manifold can cause air leaks if they’re cracked or the clamps are loose

Reeds are worn out (2-stroke)

Although not as likely, the reeds on a 2 stroke can cause idling problems. The reeds aren’t much harder to remove than the carb.

Once you have the reeds cage off, visually inspect the reed petals. If you hold it up to light and there’s a gap, then the petals are worn and should be replaced. 

Worn 2 stroke reeds Dirt Bike Won’t Stay Running: How Easily To Make It Idle
2 stroke reeds that are cracked/worn

Carburetor Is Worn/Broken Internally

Sometimes there’s just no other explanation than that the carburetor itself is causing the problem. Whether your dirt bike won’t stay running, or it runs rough, internal wear or damage can wreak havoc on your riding time. 

In an attempt to save money, find the same bike as yours with a known working carburetor. If you’ve tried every other jetting change, swap your carb with it and see if your problem is fixed.

Yes? Then replace the carb and get back to having fun riding again.

Stator Is Bad

A faulty stator is another less common problem, but still occasionally happens, and that’s why we’re covering it here.

You may be able to get your bike started with a bad stator, but it won’t be able to charge and power the electrical system and/or battery. 

XR600 Stator Dirt Bike Won’t Stay Running: How Easily To Make It Idle
A look behind the stator cover of a Honda XR600

How To Test A Dirt Bike Stator

A way to test your stator is to use a voltmeter. Check the voltage of your battery with the engine and power off. It should check around 12.5-13 volts. If it’s lower than that, your battery needs to be charged or possibly replaced.

Start the engine and check the voltage at the battery again. Rev up the engine RPM and you should see the voltage increase above 14 volts. If the voltage does not increase then the stator is not charging the battery.

This either means that the stator is bad, or that you simply have a poor or missing electrical connection/ground.

Scooter or mini bike won’t idle

Maybe you have a scooter motorcycle or a mini bike that won’t idle. Has it been sitting for a few months or years? Most likely, the carburetor is dirty and needs to be cleaned.

I always recommend using fresh gas if your motorcycle, dirt bike or ATV has been sitting for a while. Pump gas breaks down and gums up very quickly these days, causing major problems that can cause your dirt bike to not start or stay running.

Bottom line: why your dirt bike won’t idle

Remember to always start with the easy things – is it getting enough gas and air?

Here are the top 13 reasons why your dirt bike won’t idle:

  • Idle set too low
  • Engine isn’t warm/didn’t leave choke on long enough
  • Fuel/air screw needs adjustment
  • Pilot jet too large
  • Pilot jet or circuit is dirty
  • EFI injector is dirty
  • Dirty air filter
  • Fuel filter clogged/line kinked
  • Float level set too low
  • Air leak
  • Reeds are worn out (2-stroke)
  • Carburetor is worn/broken internally
  • Stator is bad

How to become a better rider after fixing your bike

Now that your dirt bike is idling properly, it’s time to improve your riding skills so that you can ride off-road with confidence and not get hurt from riding over your skill level. Click here to learn the basic riding techniques that give you more control.

Joe Shmoe

Thursday 6th of January 2022

so then how do i find out which one it is i set the idle high but the engine runs high then low then high then low until it finally dies. Idk what to do

Kelley Fager

Saturday 8th of January 2022

Hey Joe, it sounds like your jetting might be lean, or there could be an air leak. It's hard to know without more info and what bike it is. The carb could also be dirty if it's been sitting for a while.