How To Accurately Diagnose
Why Any Dirt Bike Won't Start

If your dirt bike won't start or stay running, it's known by all how frustrating and embarrassing this can be. Even more so after you've run the battery down or tried push starting it, yet it still won't start and run properly, all the while your riding buddies are waiting on you to get that thing started.

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 we're going to go over the most common reasons why dirt bikes don't start (barring any catastrophic engine damage), and show you some real world tips that anyone can do that'll get the most stubborn motorcycle back to running reliably.

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 Motorcycles Don't Start

Its Dead or Just Clicks When I Push the Start Button

Picture of an Electric Start Button

If the motorcycle is equipped with electric start and it appears lifeless, you'll want to first check whether or not the battery or electrical system is completely dead which is easiest accomplished by checking for headlight or taillight illumination and intensity, as well as whether the horn or any other accessories work. Alternatively, the battery's state of charge can be tested using a battery load tester ideally, however for the DIY home mechanic a Digital Volt & Ohm Meter (DVOM) or even a cheap test light will suffice and let you know if the battery has voltage available.

If the battery even as much as illuminates a test light (or headlamp), or makes your DVOM display close to 12 volts 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 Air, Fuel, Compression and Spark 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, I can assure you that this is not why it's not starting so do NOT flip them over. 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 DVOM 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.

First Of All, Let's Make Sure The Engine Really Isn't Getting Any Fuel

If you haven't personally seen fuel being sprayed from the injector nozzle (Which is a tiny amount), then due to the complexity of a fuel injected motorcycle, it is not uncommon to misdiagnose a fuel injection system. Even the pro's do, and if you don't have money to burn on unnecessary parts follow along...

Before beginning any sort of elaborate diagnosis on the operation of the motorcycle's electrical or fuel pressurization system, it's very wise to manually introduce some fuel to the engine (NOT Starting Fluid) by directly accessing the intake tract which is easily accomplished by removing the air filter and squirting some fuel into the intake using an emptied soda bottle with gas in it and a small hole poked in the lid and attempting to start the engine.

It Starts
If the motorcycle starts and dies when manually introducing fuel to it, this confirms that the engine is in good enough condition to run and that the ignition system is performing and that the issue is truly related to no fuel being delivered at the injector(s)
It Still Won't Start
If you're sure you got some gas in the intake and yet it still won't start, This would indicate that your problem is not fuel related and more likely that you have a sudden electrical issue which will be easiest diagnosed with a factory service manual specific to the motorcycle, or possibly even mechanical issues which may become evident with a compression test.
  • 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. Both of which are discussed in greater detail below.

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 DVOM 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.

Warning: Ignition Systems Produce Extremely High Voltage

Ignition systems produce voltage in the THOUSANDS of volts so its advised to NOT be in contact with the ignition system when attempting to start a dirt bike.

Although the shock is DC and generally harmless, If you get "hit" by the secondary ignition system via a primary or kill switch wire, spark plug wire, a coil, or a coil on plug setup, you'll find that you'll let go in an INSTANT, but getting shocked is easily avoidable 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 (Capacitive 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.
Aftermarket Accessories Improperly or Poorly Hooked Up
Aftermarket alarms or remote start kits are especially notorious for causing a no start condition, so if applicable, reconnect any previously cut OEM wires to OEM spec, then attempt to start the bike without the aftermarket device connected.
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 so as to ensure that the engine turns freely.

I Have Power At the Battery But the Bike Is Dead

So pressing the electric start does nothing at all or little more than click but you're sure you've got power at the battery, or even better, the headlight or taillight brightly illuminates when you turn the bike on, we'll be able to get a rough idea as to the battery's amperage capabilities, as well as the integrity of the electrical system (ie: tightness of connections) as all too often electric start bikes don't start due to loose or corroded connections so we'll be looking into this first.

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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Profile Pic Guest October 18, 2021

i have a apollo 250 rx and all eletrical componets are working and in good shape all wires in good shape as well and when i hit the eletric start button it is dead, battery has good charge.any help?

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Profile Pic Admin October 19, 2021

First ensure that both battery cables are tight at both ends, then using a test light check for a start signal at the starter solenoid on the small wire(s). If all that checks out see if the test light illuminates at the starter when you press the start button as this will determine if the starter solenoid or starter is functioning.

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Profile Pic Guest August 29, 2021

Son was riding his 2005 Yamaha YZ250 It was running and mid ride it just stopped on him. The kick start is now way too hard to kick it, you can barely kick it down it way to hard/tight what could be wrong with it? Please help

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Profile Pic Admin August 29, 2021

Sounds like the piston, rod or crank bearings seized. Id check the fuel he was using and ensure it had the proper premix. Additionally, Id pull the top end off and inspect the piston skirt for any seizure scoring, as well as ensuring that the piston and connecting rod bottom bearing moves freely, as well as ensuring that the crank turns freely.

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Profile Pic Guest August 23, 2021

How about a brand new rfz 125cc won't kick over as in the kickstart is stuck. Goes down about 3 inches and stops.

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Profile Pic Admin August 26, 2021

Sounds like it's locked up. Try removing the spark plug and kicking it over.

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Profile Pic Guest August 23, 2021

Hey I have a low hour well maintained with 8 hours 2016 kx250f. It takes roughly around 10-20 kicks to start the bike which is very unusual. Even after priming it and kicking TDC every time. I’m just trying to narrow down if it’s the condenser or the regulator I’m not sure how to test those. But I’ve ran through injectors and all the wiring. If you could help that would be awesome, thanks

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Profile Pic Admin August 26, 2021

Id perform a compression test on it

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Profile Pic Guest August 24, 2021

Should be mint. I’ve replaced the valves thinking there was rust on seats but everything checked out went down inspected the piston and cylinder and put it all back together with oem new rings on top of port and polish. I’m lost for words. Ran a test on the fuel pump twice fuel pump is good

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Profile Pic Guest August 18, 2021

I have honda crf250r 2012. How many volts will have to be at fuel pump while starting the bike. At condenser it´s only 4 wolts while kickstarting the bike. What could be the problem?

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Profile Pic Admin August 18, 2021

It will need atleast 10.5 volts at the fuel pump when kick starting it. I'd unplug the fuel pump and check the voltage at the plug while kick starting it as this will eliminate the possibility of the fuel pump being internally shorted causing a voltage drop. Also check all electrical connections to ensure they are all clean and tight.

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Profile Pic Guest April 30, 2021

My 2009 kx450f wil not start hot or cold , it’s fi , i can spray brake cleaner in it and it will run all day with no issues, very fast and responsive

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Profile Pic Admin April 30, 2021

Try unplugging the injector and checking for a pulsating light during cranking with a noid light available at most auto parts stores. You should have power on one side and a pulsating ground coming from the ECM on the other. Also try unplugging the ECM and reconnecting it while making sure all terminals look good and are not pushed out of the connector.

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