How To Diagnose
Why A Dirt Bike Won't Start

If you're left with a dirt bike that won't start or stay running, it's known by all of us how frustrating, exhausting and embarrassing this can get. Also, most of us know how pathetic it looks to push, or even more entertaining; To pull start a dirt bike in an attempt to get it running after you've already run the battery down or leaned the bike against something so you can deliver a "better" kick.

Even a nice ride that occasionally has starting or staying running issues can start to seem like a POS, although a dirt bike that has starting or staying running issues, usually has an easily correctable condition that's easily diagnosable using basic tools and a systematic approach, so below, we're going to go over the most common reasons why dirt bikes don't start (barring any catastrophic engine damage), and provide tips on what you can do towards getting a motorcycle running.

If it's electric start and nothing happens, or it just "clicks" when you press the start button, Our section on what to do when it's dead or just clicks should be a must see before doing anything else.

The Basics Of Why Any Motorcycle Won't Start


Its Dead or Just Clicks When I Push the Button

If the motorcycle is equipped with electric start and it appears lifeless, you'll want to first check the battery's state of charge using a battery load tester ideally, however for the DIY Home mechanic a test light or DVM will suffice and let you know if the battery has voltage available, however it requires specialized knowledge in use of a DVM to determine amperage capabilities and we're focusing on the basics anyone can perform with ease.

If the battery even as much as illuminates a test light, or makes your DVM display close to 12V at the battery we need to find out how far the battery power is reaching by ensuring the key (if present) and any "Run/Off" switches are on, followed by ensuring that all connections at the battery and starter solenoid are clean, secure, undamaged and tight, then be sure to review the tips on How To Diagnose Why My E Start Bike Wont Start.

It Turns Over But Wont Start

Presuming its kick start or the E start is working and the engine is turning over, the first thing to do when diagnosing why any dirt bike won't start, is to ensure the 4 basic necessities listed below are met. Any dirt bike, whether it be a 2 stroke or 4 stroke, MUST have these 4 prerequisites to start and run:

Always Keep These 4 Prerequisites In Mind

Keep in mind during your diagnosis that everything besides these 4 necessities to the right is more less just along for the ride when it comes down to a dirt bike running, or not running but this doesn't mean to pull the pipe or intake boot off in a process of elimination as there's not generally much to see, or need to remove these components for proper diagnosis of a "No Start" condition, nor when diagnosing why a motorcycle won't run (or stay running) or is routinely hard to start.

  1. Air - This means a clean air filter. A dirty air filter can easily result in a fouled spark plug, and If you haven't seen the air filter in a while, be sure to check out our article detailing: How to Clean and Maintain an Air Filter on a Dirt Bike, but also be sure to review the air tract evaluation tips below as this will help in evaluating the condition of the air filter and intake tract.
  2. Fuel - Presuming there's gas in the tank (you can hear it sloshing, or you might've even looked inside) you need to ensure there's an adequate supply of clean fuel reaching the carburetor or fuel injector(s) which there is more below on fuel quality and fuel supply to the engine including specific instructions for diagnosing a carbureted motorcycle, as well as how to diagnose the fuel injection system on fuel injected bikes so those are good places to start if you're suspecting that the engine's not getting fuel.
  3. Compression - If you've got a dirt bike with little or no compression, you're wasting your time kicking or running the battery down and swapping spark plugs. Little or no compression generally means the top end is either worn out, or there's a leak around a sealing surface. If you're interested in learning about a compression test, you should take a look at the article elsewhere on Performing a Compression Test for more info on checking the top ends condition.
  4. Spark - This is usually the first thing that everyone suspects when a dirt bike won't start, but this can also be misleading because a plug will frequently fire out of the cylinder, but under the pressure of being in a cylinder, a spark plug will frequently fail to fire and light the air / fuel mixture. If you have doubts about the spark plug firing, put a new spark plug in the hole and see what happens, then if it still won't start, Check out our other tips on determining why the bike isn't firing.

Wisdom Regarding Reed Valves

If you're kicking a 2 stroke to death, or running the battery down repeatedly and swapping spark plugs but the dirt bike won't start, or is extremely hard to start and the bike has a reed block, the reeds may be in a less than ideal condition, as the reed petals do get worn on 2 strokes which results in the petals becoming flayed, broken, chipped, split, cracked or just not sealing well to the reed block but these conditions generally result in poor performance more so than leaving you left with a dirt bike that won't start.

Should you find that the reeds are lifted and not sealing flat, do NOT flip them. Worn reeds should only be replaced, and never flipped otherwise the engine may be inhaling parts of the reed petal(s).

More Tips on What to Do When A Dirt Bike Won't Start

More About the Air Filter and the Intake Tract

If you happened to get called out on a nasty looking air filter, you may want to replace the spark plug while you're giving the dirt bike some needed attention. Since you're the one with a bike that won't start and the problem may be related to the engine having been unable to breathe, after this startling revelation of how the air filter looks, a new spark plug should be a must, along with a Compression Test while the spark plug is out just to check out the condition of the top end after running with a choked up, dirty or worse; Torn air filter allowing dirt and particles to reach the engine which will greatly accelerate the wear of internal parts resulting in low compression and lots of damage.

The safest bet for getting the most longevity out of any engine is to keep an eye on the air filter and check out the article on: Keeping your air filter and intake clean as if the motor depends upon it.


It's Carbureted and Gas Runs Out of the Hose but The Engine isn't Getting Gas

If you're pretty sure there is no gas reaching the engine, the carburetor likely needs a full disassembly and cleaning but before pulling the carb off there are a couple things you can check to see just how far through the carb the gas is making it.


  • First ensure the dirt bike is clean before beginning any service.
  • Once the motorcycle is clean, and the fuel is turned off, carefully remove the fuel sediment bowl that is threaded into the bottom of the carburetor's float bowl, or loosen the drain screw to allow fuel to come out and be able to get an idea of the condition of the fuel in the carburetor.
  • If it has a sediment bowl and the sediment bowl looks anything but clean, its likely that the carburetor is going to need a full disassembly and cleaning.
  • If the sediment bowl looks clean, with the sediment bowl removed, place a folded rag below the carburetor and briefly turn the gas on and almost immediately back to off. At this time fuel should flow from within the carburetor and run out the bottom of the carburetor where the sediment bowl generally resides. If no fuel comes out now, it's likely that the needle and seat in the carburetor are sticking so you'll likely want to take a deeper look with a full disassembly of the carburetor for a full cleaning.

Here's how to rebuild a carburetor yourself, although if you're hesitant, I assure you that a complete disassembly and cleaning of a motorcycle carburetor is not difficult, although you may want to grab a service manual specific to the bike you're working on for specifics related to the motorcycle being serviced.

It's Fuel Injected but The Engine Is Not Getting Fuel

Fuel injected bikes are not difficult to diagnose and often perform flawlessly with minimal maintenance, however since off-road motorcycles are subjected to some pretty harsh conditions, problems with the FI system on a motorcycle can develop (which are usually electrical related) and can be easily diagnosed at home with a cheap DVM or a basic 12V DC test light following the guidance below, although a factory service manual specific to the motorcycle will provide invaluable wiring schematics and additional "built in" on board diagnostic procedures specific to the motorcycle.

  • As with a carbureted bike ensure the motorcycle is clean before beginning service.
  • Once the motorcycle is clean, we can begin diagnosis of the motorcycle's EFI system while keeping in mind that the 2 prerequisites for any fuel injected system to perform properly are are fuel pressure and injector signal.

With the vast assortment of FI dirt bikes from battery-less kick start models, to battery equipped, electric start models, were gonna focus on what the fuel injection system requires for the motorcycle to run with less emphasis on model specific details.

Does it Have Fuel Pressure?

Fuel injected motorcycles rely upon a fuel system that is pressurized by an electric fuel pump. All electrical connections within the fuel pump's electrical circuit must be clean and tight to allow sufficient current (amperage) to the pump for it to be able to develop and maintain adequate fuel system pressure.

Check the fuel pump for power and a ground with a DVM or test light by back probing the electrical connector nearest the fuel pump. Should power or ground be absent, check all the motorcycle's fuses, followed by a thorough inspection of all the wiring under the gas tank, seat or side / front panels / number plates while looking for any chaffing or other damage to the harness, as well as ensuring no electrical connectors have become partially separated.

If you have power and a ground to the fuel pump, but the pump is not running, as a diagnostic aid for motorcycles with an in-tank fuel pump, it is often possible to forcefully strike the gas tank nearest the fuel pump with the heel of your palm or a mallet and get an in tank pump to work temporarily, however if this gets the bike running, it is strongly advised that you replace the intank fuel pump immediately.

Does it Have Injector Power And A Ground Signal?

As part of your preliminary diagnosis, you'll want to ensure that the electrical connector at the fuel injector(s) are undamaged, secure and have 12V of continuous power to one side of the plug and a pulsing ground to the other, anytime the engine is being cranked as the injector(s) are frequently powered by a constant power supply while the ECM sends a ground signal to the non powered wire to control when and for how long the injector is "fired" / enabled / turned on which is controlled by the ECM and is referred to as injector pulse width.

Checking for power and ground at the injector(s) is easiest accomplished using a "Noid Light" available at most auto parts stores.

Should the noid light not flash during cranking, it will be likely that there will be no spark at the spark plug either. At this point it will be necessary to check for an independent power supply and pulsating ground at the injector with a test light. Should voltage or a pulsating ground be absent during cranking, very carefully inspect the entirety of the harness and all connectors for any sign of visible damage or electrical connectors that may not be fully seated and ensure that the electrical system is waterproof.


Does It Have Low Compression?

If the top end is worn out, or there's a gasket that's not sealing (ie: blown head gasket, leaking base gasket) the motor could be low on compression, and low compression certainly will make an engine hard to start, if you can get it running at all.

If you're suspecting low compression, be sure to check out our article on performing a compression test as doing so will give you an idea of the condition of the top end, at which time, If the top end proves to be in need of service, be sure to grab a factory service manual specific to the bike you're working on, and on the way back together, be sure to stay on top of oil changes and air filter services to prolong the life of the top end.


There's No Spark

Got a clean air filter, clean gas in the tank that's reaching the engine and good compression? After you've been over everything listed above and have come to the conclusion that there's no spark, there are a few common reasons for a no spark condition which you should check before taking on a complex electronic ignition system.

Remember: Ignition Systems Produce High Voltage

Ignition systems produce extremely high voltage so its not advised to be in contact with the ignition system while attempting to start the dirt bike.
Although the shock is generally harmless, If you get hit by a plug wire, a coil, or a coil on plug setup, you'll find that you'll let go in an instant but this is best avoided by not holding or touching ignition components during cranking or running of a motorcycle.

Here's the Most Common Reasons a Dirt Bike Loses Spark:

  • Worn Through, Chafed & Smashed Wires or Broken Insulation
    Picture of broken insulation exposing bare wire The wires that run to the kill switch or other handlebar controls, as well as the wires which run to control units and other electrical components are frequently routed in such a way that they get smashed between the fork leg, or triple clamp, as well being routed in areas susceptible to chafing and on older dirt bike's it's not uncommon to see broken insulation exposing bare wire which could and often does result in a short or open circuit resulting in a dirt bike that suddenly loses spark.
    Just remember; The harsh conditions that dirt bikes are exposed to can cause an open or grounded circuit almost anywhere along a harness that will easily create a no spark condition, so be sure to take a good look around the steering stem, and the area under the fuel tank for smashed or otherwise damaged wires or insulation.
  • Poor Electrical Connections
    Poor connections are another one of the common reasons why a dirt bike won't start. One way to check for this is to get a visual of every wire where it connects, or is secured, then ensure with your finger that the connection looks and feels tight by trying to move the connector, or to even be sure that plug style connectors are secure and look undamaged. Occasionally you'll find that wires secured with a nut or screw may have worked themselves loose, the terminal is loose feeling (worn out), or the plastic locks being broken allowing ECU's (Engine Control Units), CDI's (Capacative Discharge Ignitions) and other components, or their connectors to develop a loose connection.

    Additionally, look for electrical terminals which are corroded, stretched out from probing and not making a tight connection to the male counterpart, as well as you'll want to keep an eye out for evidence of a previous repair that wasn't performed properly. During diagnosis of the electrical system, be sure to inspect the entirety of the electrical system before condoning an expensive electrical part, as you may be surprised at what poor, or broken electrical connections can cause.
  • A Kill Switch Going Bad
    Kill Switches DO go bad. If you're not sure about whether or not the kill switch is causing the dirt bike to not start, try unplugging it and see if the dirt bike will start. (Note: Some dirt bikes won't start without the handlebar controls plugged in but this gets into a higher level of diagnosis that this article doesn't cover, but which can be reached by reviewing a service manual specific to the dirt bike you're working on for diagnostic flow charts necessary for properly diagnosing why a dirt bike wont start.

Why Won't My Electric Start Bike Start

If you're left with an electric start motorcycle that appears lifeless but has a backup kick starter, by all means you should attempt to start it with the kick start.

If the motorcycle does not have a kick start backup, it may be possible to push start the motorcycle.

If you don't have the luxury of a backup kick starter, nor you're in a position to push start the motorcycle, ensure all electrical connections are clean and tight, then check all the fuses. If these tips do not resolve your no start condition, it will be necessary to refer to a factory service manual specific to the motorcycle for additional manufacturer specific diagnostic procedures.

Need Parts? Check out These New, Used, OEM & Aftermarket Parts

If you've found the problem, corrected it and want to ensure your dirt bike will stay running in wet environments such as washings and water crossings, or the like; Check out our article on: Waterproofing the Electrical System

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