How To Diagnose
Why A Dirt Bike Won't Start

If you still have a carburetor under the seat and you're left with a dirt bike that won't start, 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 put it on the stand, or hooked a strap from the handlebars to the side of the truck so you can deliver a "better" kick.

Even a nice ride that occasionally has starting issues can start to seem like a Real POS, but 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.

What Does A Dirt Bike Need to Run?

The first thing to do when diagnosing why a 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 Basics In Mind First

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 simply 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 going to the carburetor which there is more below on fuel quality and fuel supply to the engine so that's a good place to start if you're suspecting that the motor'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.

A Word About 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.

The Gas Runs Out of the Hose but The Engine isn't Getting Gas

This could easily get into diagnosis deeper than this article is intended to reach but if you're pretty sure there is no gas reaching the motor, the carburetor likely needs a full disassembly and cleaning but before pulling the carb off the motor 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
  • Once the motorcycle is clean, and the fuel is turned off, you'll want to carefully remove the fuel sediment bowl that is threaded into the bottom of the carburetor's float bowl to get an idea of the condition of the fuel in the carburetor.
  • If the inside of 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 back on but only for a second. 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 can assure you that a complete disassembly and cleaning of the carburetor is not difficult, but you may want to grab a service manual specific to the bike you're working on for specifics related to the motorcycle being serviced.

Have you Got 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 carb 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:

  • 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, by all means unplug it and see if the dirt bike will start. (Note: Some dirt bikes wont 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.
  • 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.
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 watercrossings, or the like; Check out our article on: Waterproofing the Electrical System

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