13 Reasons Why Your Dirt Bike Won't Start & How To Fix It

Wondering why your dirt bike won't start, what it means, and how to get it running? Whether you just pulled the bike out of a barn after it's been sitting for years, it just randomly quit running, or somewhere in between, you're in the right place!

In this article, I'm going to share with you all of the most common reasons why your dirt bike won't start (Free checklist), how to fix it so you can go riding, and some easy tips to prevent it from happening again!

These tips and tricks are based on my 20+ years of experience owning, riding, and working on dozens of different dirt bikes, as well as street bikes.

Why Won't My Dirt Bike Start?

In order for your dirt bike to start it needs spark, a proper mixture of air and fuel, and enough compression from the engine. If your dirt bike is not getting just one of these, you can kick it over all day long and it still won't start.

To save some possible time, we'll take a quick look at each of these areas to see if you can spot something simple.

That way you won't spend an hour trying to fix one thing when the problem could be something completely different.

Oxygen - Your Dirt bike Needs Air

Air comes first, so pop the side cover and/or seat off and take a look at the air filter. Is there a complete air filter there?
Mice can easily get into the airbox during storage and make a nice "welcome nest" for you.

It's also easy to stuff a rag in there while working on the bike and forget to remove it when you put everything back together.

All I can say is, stranger things have happened, and it's always safe to check first, especially since it only takes a minute.

A Clean Filter Is A Happy Filter

If the air filter is there, how dirty is it? If it's caked with sand and mud, that alone could be causing your dirt bike to no run.

Clean or replace the filter and try starting it again.

Are You Leaking?

A less-common, yet possible cause, could be an air-leak in the system. Check the intake boot for cracks, as well as any bolts or gaskets in between the airbox, carburetor, and engine.

Over time, this rubber gets hard and will crack. It's not always easy to see it, but if you wiggle the carb or spray some brake cleaner around it with the engine idling, the RPM will change.

You can also see in the picture below that the carb isn't fully seated on the intake manifold. It should be fully pressed in or dirty and air can easily get into the engine and cause major starting and running problems.

2 Stroke Reeds Leaking Air

On a 2 stroke engine, the reed valve has pedals that wear out over time. They can last hundreds of hours if the bike isn't ridden hard, but if the edges are chipped off, it could let unwanted air through and not allow the bike to start.

This takes longer to remove and inspect, but it's just one of the many reasons why your dirt bike won't start. Below shows the edge of a reed petal chipped - this needs to be replaced if you want your 2 stroke dirt bike to start and run properly!

Fuel - Not Enough or Too Much?

The engine requires air and fuel in order for combustion to occur, so the next step is to make sure the engine is getting gas/premix. The first place to check is the gas tank.

Is the tank clean with fresh gas? Gas or premix that has sat for a number of weeks will degrade and start to gum up.

If the gas smells like old paint, dump it out before doing anything and clean the tank out.

Don't forget to check the tank filter!

Fuel must go through the petcock to get to the carburetor. If the tank is empty, now is a good time to take a peek at the seal and filter screen on it.

Replace them if they are damaged, leak, have deteriorated, or are missing with new parts/assembly. A gas line that is cracked or plugged needs to be replaced as well.

Are You Jetted The Right Way?

A dirt bike with poor jetting can not only prevent it from starting, but it won't run well and will do more damage over time. Rich jetting will cause the engine to flood over easily.

Flooding the engine is when too much fuel gets into the cylinder/combustion chamber. Too much fuel and not enough air and you'll just be kicking it over with no success.

What to do if the carb/engine is flooded

Continuous start attempts will also flood the engine with even more fuel, making your situation worse. If the engine is flooded, it's best to wait for the fuel to evaporate.

You can try starting it again after sitting for at least a few minutes. If it backfires, then there's still most likely too much fuel.

How to know if the jetting is too lean

Your carb may just be dirty if it's hard to start when cold, but lean jetting can also cause this. A common symptom of lean jetting is if your dirt bike is always harder to start when cold, but easier when the engine is fully warmed up or in warmer weather.

Don't worry, it's usually an easy fix and you might be able to do it in a few minutes with my free guide. If you have a 2 stroke then click here to learn fuel screw adjusting and here for a 4-stroke fuel screw tuning guide.

How to start a dirt bike in the cold

The process for kick starting a dirt bike in the cold is the same as in warm weather, but there's a couple extra things you can or need to do to make it easier. 

For one, the carb jetting needs to be optimal. If your dirt bike started easily in warm weather, then you'll need to go richer on the jetting. The colder air requires more fuel in the mixture to burn properly. 

Starting a 2 stroke in cold weather

The key factors to starting a 2 stroke in the cold are: use the choke, richen the jetting if needed, and let it warm up longer before riding hard. 

Starting a 4 stroke in cold weather

A 4 stroke can be even harder to start in the cold because of the high compression, depending on which bike you have. 

Always use the choke when the engine is cold, but if your 4 stroke dirt bike carb has an accelerator pump, you have advantage. You can give the throttle a 2-3 twists and the accelerator pump will squirt a small shock of gas through the carb. This will richen the mixture even more, making it easier to start in cold weather. 

2 stroke has fuel and spark but won't start

You've confirmed that the gas is getting to the carb, and the spark plug is properly igniting, but your 2 stroke dirt bike won't start. The most common causes are a dirty carb or low compression.

Keep reading to learn how to properly fix a dirty carb. Low compression is the result of a worn top end.

Battery Just Clicks?

Electric start dirt bikes have a battery that powers the starter. If the battery has a low voltage or charge then it may not have enough power to start the bike.

It will either spin the engine over slowly or you will hearing a "clicking" noise from the battery. This means it needs to be charged or replaced.

If you don't ride very often and your dirt bike doesn't have a back-up kick starter, you should consider getting a battery tender (Amazon). They're cheaper than a new battery and you can use it on any dirt bike or recreational vehicle in your garage.

A battery tender is better than a charger because it won't overcharge the battery, which may ruin it or shorten its life. Lithium batteries require a Lithium specific battery tender (Amazon).

Won't Start When Hot

A dirt bike that won't start when hot is often a sign of the engine getting flooded. A rich jetting mixture is one of the main causes for a flooded engine.

The pilot jet is the most important jetting circuit when it comes to starting your bike. To learn more about how jetting works and to get your dirt bike running better go here.

Not Too Rich, Not Too Lean

On the other hand, if the jetting is too lean then the engine isn't getting enough gas to start it. Try adjusting the air screw or fuel screw on the carb to allow more gas through the carb.

If that doesn't work then the pilot jet may need to be swapped for a larger size.

Use Quality Gas

Next up is the carburetor, which is the root cause of many 'no-start' situations. Dirt bikes with smaller carbs, jets and passages can gum up in a matter of weeks because of old gas sitting in it.

Even if you can see through the pilot jet, there may be just enough crud stuck on it to not allow enough gas through to ignite the engine.

Using high quality gas will make your dirt bike run better and cause less problems in the long run.

Why most pump gas is so bad in your dirt bike

Regular pump gas is full of so many additives these days that break down in a short time. Once the gas breaks down it will cling to your carb and clog the jets and passages.

A good quality gas will have no ethanol in it, so look for that on the pump when filling your gas can. Non-oxygenated gas works very well in dirt bikes and other performance machines.

It will run the most consistently and last longer than regular pump gas before breaking down and going bad.

Cleaning The Carb

Carb cleaner and compressed air are your friends if the carburetor isn't too filthy. You can sometimes just loosen up the carb clamps and rotate it to spray out the jets while still on the bike.

This can be done in a matter of minutes by removing the float bowl on the bottom of the carburetor.

Confirming Your Carb Needs A Thorough Cleaning...

If that doesn't work, you may need to take the carburetor completely off and disassemble it for a more thorough cleaning. There are a couple more quick ways to confirm that the carb may be dirty.

With starting fluid or a little bit of gas (half a spoonful is enough) down the spark plug hole try starting it again.

If it starts or runs for a second or two then you know it's not getting gas from the dirty carb. You can also try and push start your dirt bike.

The Best Way To Clean A Carburetor

I have cleaned many dirt bike carbs over the years, and the best addition to my tool crib for this was an ultrasonic carb cleaner. For less than 100 bucks you can clean all the dirty carbs for years to come without having to pay a shop to do it.

An ultrasonic cleaner penetrates all the old gas and dirt that gets stuck in the tiny passages that are sometimes impossible to clean with just carb cleaner spray (which I rarely ever use anyway).

It will pay for itself after cleaning just one or two carbs. If the carb is really dirty, then your dirt bike will start easier, idle better, and run stronger with better throttle response after a deep cleaning with the right solution. To buy your ultrasonic cleaner on Amazon click here or the image below.

I always use Simple Green Industrial cleaner & degreaser (Amazon) because it's strong enough to clean old gas and it won't hurt your carburetor.

If neither of those work, it may not be a fuel problem after all, but lets move on to one more quick check if cleaning the carburetor isn't your solution.

Does your dirt bike have spark?

Your dirt bike may be getting air and fuel, but if there's no spark, it won't even want to start.
A quick way how to check if a dirt bike has spark:

1. Remove the spark plug from the engine.
2. Put the cap back on the spark plug.
3. Resting the spark plug on the engine .
4. Slowly kick/turn the engine over.
5. You may need to turn off the garage lights to see it.
6. Look for a small, blueish spark of electricity on the end of the spark plug.

If you don't see anything, be prepared to spend some time swapping out parts if you don't want to replace everything in the electrical system.

Troubleshooting No Spark

If you know someone with the same bike, ask if you could temporarily swap some electrical parts off of theirs. It could be as simple as a faulty kill switch, or a bad ground in the system.

However, you may have to swap out the CDI box or even the stator to find the root problem.

Once you find the part causing the problem, order a new one and give your generous friend back his/her parts, as well as taking them out to lunch if they lent you a hand in your project because they could have saved you a lot of money by not ordering the wrong parts.

Finally get your dirt bike started but now it won't stay running? Click here to find out why!

My Dirt Bike Still Won't Start!

If you still can't figure out why your dirt bike won't start, there's a good chance that the engine needs some attention. Parts wear out after many hours on them, and a worn out engine can cause a dirt bike not to start or run well.

Low compression is common on motocross bikes that have been ridden for years. If you check the engine compression when the engine is new, you will be able to tell when it needs to be rebuilt when the compression goes more than 25% below what it started at. For all the symptoms that lead to a top-end needing a rebuild read more here.

The Most Common Reasons Why Your Dirt Bike Won't Start:

• Gas isn't turned on/no gas in tank
• Gas is old and varnished
• Choke isn't on/not working properly
• Carburetor is dirty
• Fuel injector is clogged/dirty
• Air filter is dirty
• Poorly tuned carb jetting
• Battery is too weak
• No spark
• Spark plug is fouled
• Faulty/broken kill switch
• Intake air leak
• Low compression

Put this cheat sheet in your garage for when your bike won't start!

I want to give you this checklist as a FREE 1 page PDF that you can print and pin up in your garage so that you can refer to it any time your dirt bike won't start. Click here to download it for free.