1. Fuel Delivery Issues (Beyond Just Getting Fuel *To* the Engine):
* Carburetor Problems: This is the most likely culprit. A dirty carburetor is very common on older ATVs. It might be clogged, have a stuck float, or need adjustment. Consider:
* Cleaning the carburetor: This often requires disassembly, cleaning with carburetor cleaner, and possibly a rebuild kit if parts are worn. This is a common fix.
* Fuel filter: A clogged fuel filter will restrict fuel flow, even if the tank is full. Replace it.
* Fuel lines: Check for kinks, cracks, or blockages in the fuel lines.
* Fuel pump (if equipped): Some models have fuel pumps; make sure it's working. Listen for a humming sound when the key is turned on.
* Vacuum leaks: Leaks in the intake manifold or boots can disrupt the air/fuel mixture.
2. Ignition System Issues (Despite Having Spark):
* Weak Spark: Even if you *see* spark, it might not be strong enough to ignite the fuel/air mixture reliably. A new spark plug (correct heat range) is a cheap test.
* Timing: The ignition timing could be off. This is more complex to diagnose and usually requires a timing light.
* Stator/Coil Problems: These components generate the spark. A failing stator or coil can produce weak or intermittent spark.
3. Engine Mechanical Issues:
* Valve Adjustment: Incorrect valve clearances can prevent proper combustion.
* Camshaft/Crankshaft Issues: While less likely, internal engine damage could prevent proper operation even with compression.
4. Other Potential Problems:
* Kill Switch: Make absolutely sure the kill switch isn't accidentally engaged.
* Battery: A weak battery might not provide enough power to the ignition system, even if the spark appears okay. Test the battery voltage.
* Wiring Issues: Check all wiring connections for corrosion or loose wires. A bad connection anywhere in the electrical system can prevent starting.
Troubleshooting Steps:
1. Start with the simplest things: Check the kill switch, battery voltage, and fuel level.
2. Inspect the spark plug: Is it wet with fuel? If so, you have fuel getting to the cylinder. If it's dry, you might have a fuel delivery problem. Replace the plug regardless – it's cheap.
3. Examine the carburetor: This is the most likely culprit for starting issues. Consider cleaning or replacing it.
4. Check fuel lines and filter: Ensure there are no blockages.
5. If you suspect ignition problems: Consider checking the stator and coil (this is more advanced).
If you're not comfortable working on the carburetor or other engine components, take it to a qualified ATV mechanic. Trying to diagnose electrical problems without the right tools and knowledge can be frustrating and potentially damage the engine. A professional can quickly pinpoint the problem.
