My Dirt Bike Won’t Run – Diagnosing A No-Start

Any day your dirt bike won’t start is a bad day. The problem could be as simple as turning on the gas. However, if regular maintenance hasn’t been performed, you could have a much bigger underlying issue. Not changing the oil in 20 some hours can do a good deal of damage to the engine. If you just haven’t ridden the bike for a few months and it’s just been sitting in the garage since you last rode it, you can often find and solve the problem in less than an hour.

In order an engine to run, it needs air, fuel, and spark. If your dirt bike is not getting just one of these, you can kick it over all day long and it won’t start. In order to save some possible time, we’ll take a quick look at each of these areas to see if we can spot something simple. That way we won’t spend an hour trying to fix one thing when the problem could be something completely different.


Air comes first, so pop the side cover and/or seat off and take a look at the air filter. This may be a dumb question, but is there a complete air filter there? It’s easy to stuff a rag in there while working on the bike and forget about it when you go to 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. If the air filter is there, how dirty is it? If it’s caked with sand and mud, that alone could be preventing your no-start problem. Clean the filter and try starting it again.

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. 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 things that can cause a no-start dilemma.

This filter looks a little dirty...
This filter looks a little dirty…


The engine requires air and fuel in order for combustion to occur, so the next step is make sure the engine is getting gas/premix. 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. Fuel must go through the petcock to get to the carburetor, so if the tank is empty, now is a good time to take a peek at the seal and filter on it. If they are damaged, leak, or clogged, replace with new parts/assembly. If the gas line is cracked or plugged, replace it as well.

Next up is the carburetor, which is the root cause of many ‘no-start’ situations. Especially on smaller 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 a sufficient amount of gas through to ignite the engine. Carb cleaner and compressed air are your friends if the carburetor isn’t too filthy. You can often times 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.

If that doesn’t work, you may need to take the carburetor completely off and dis-assemble it for a more thorough cleaning. There’s a couple more quick ways to confirm that the carb may be dirty. You can pour a little bit gas in the spark plug hole and kick it over. If it starts or fires 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. If neither of those work, it may not be a fuel problem after all, but lets move on to one more quick check before taking the time to clean the carburetor again.


My dirt bike may be getting air and fuel, but if there’s no spark, it won’t even want to start. A quick way to check if it has spark is by pulling the spark plug off, putting the cap back on, and resting it on the engine while slowly kicking the engine over. You may need to turn off the garage lights to see it, as it will be a small, blueish spark of electricity on the end of the spark plug. If you don’t see anything, be prepared to do spend a good amount of time swapping out parts if you don’t want to replace everything in the electrical system.

If you know someone with the same bike, ask if you could temporarily rob 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.

If you still can’t figure out why your dirt bike won’t start, there’s a good chance that it needs a rebuild if it has a lot of hours on it. 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.

If you have any questions, post a comment below… Have fun, ride safe, and keep those dirt scooters maintained!

-Tom Stark