* Compression: Low compression prevents the fuel-air mixture from igniting properly. You need a compression tester to check this. Low compression can be due to worn piston rings, a damaged cylinder head gasket, or valve issues.
* Timing: The ignition timing might be off. This is less likely if it ran recently, but a slipped timing chain or incorrectly installed components could be the culprit. This requires a more in-depth mechanical inspection.
* Camshaft: A problem with the camshaft, such as a bent lobe or incorrect timing, can prevent proper valve operation and thus combustion.
* Carburetor issues (if applicable): Even if you *think* you have fuel, there might be a problem with the carburetor's jets, floats, or diaphragm. It might be getting fuel, but not the *right* amount or at the *right* time. A thorough cleaning or rebuild might be necessary. If it's fuel-injected, then this isn't the problem.
* Air leaks: An air leak in the intake system will lean out the fuel-air mixture, preventing combustion. Inspect all rubber boots, gaskets, and the airbox for cracks or damage.
* Faulty kill switch or wiring: Ensure the kill switch is in the "run" position and that there are no broken or corroded wires in the ignition system. Check your wiring harness for any damaged wires.
* Stator Issues: While you've confirmed spark, a weak stator could still be providing insufficient power for reliable starting. This is less likely, but can be a problem.
Troubleshooting Steps:
1. Compression Test: This is the most important first step. Rent or borrow a compression tester and check the compression. Low compression points to a major engine issue.
2. Visual Inspection: Carefully inspect all the components mentioned above, paying close attention to the intake system, carburetor (if applicable), and wiring harness. Look for obvious signs of damage, leaks, or loose connections.
3. Check for Spark Strength: While you have spark, is it strong enough? A weak spark might not be enough to ignite the fuel-air mixture reliably.
4. Fuel Delivery: Even though you think you have fuel, verify fuel is flowing to the carburetor (or fuel injectors) consistently. Check fuel lines for blockages.
5. Consult a Manual: A service manual for your specific 2004 KTM 125 is invaluable. It will provide detailed diagrams and troubleshooting procedures.
If you're not comfortable performing these checks yourself, it's best to take the motorcycle to a qualified motorcycle mechanic. Diagnosing starting problems can be complex, and incorrect repairs can cause further damage.
