what causes no spark to the spark plug

First thing I'd check is the kill switch wiring. Those can get intermittent from corrosion or a loose connection. Also look at the safety switches like the tether or tip-over sensor. Bypass them one at a time to rule them out.

Start with the basics: test for power at the ignition coil primary. If you're getting 12V when the key is on, then the issue is likely on the low-tension side. If not, trace back to the CDI or pickup coil.

I had a no-spark on my 2000 Polaris Sportsman. Turned out the stator had a broken wire where it passes through the engine case. Check the resistance between the pickup coil wires coming from the stator.

Don't forget the ground connections. A bad ground to the CDI or coil will kill spark completely. Clean every connection with contact cleaner and re-tighten them.

Could also be a failed CDI unit. They can die without warning. But before buying a new one, check the pulse generator (pickup coil) resistance with a multimeter. Should usually be around 100-300 ohms.

I'm with the guy above about the kill switch. My buddy spent a week chasing a no-spark on a Yamaha Grizzly only to find a tiny crack in the kill switch housing. Replaced the switch and it fired right up.

Another common one on older Hondas are the safety switches (neutral, brake, etc.). If any of those are faulty or out of adjustment, the CDI won't fire. Try starting in neutral with the brake fully pulled.

Before you dive into electrical testing, make sure the spark plug cap is screwed on tight and the plug wire isn't frayed. Sounds simple but I've seen it fool people for hours.

I'd swap the CDI with a known good one if you can borrow one. That's a quick test. But if you don't have access, check the AC voltage from the stator's exciter coil while cranking. You should see around 100V or more.

I had a no-spark from a bad ignition coil primary. It had internal resistance of 0.1 ohms when it should have been about 0.5. A new coil fixed it. Use your multimeter on all components.

One thing that gets overlooked is the ignition switch itself. If the contacts are worn, power might not reach the CDI. Test for 12V at the CDI orange/black wire when the key is on.

I disagree with the idea that it's always the kill switch. My experience is that stators fail more often on machines with high hours. Pull the stator cover and inspect for burnt windings or oil contamination.

If this happened mid-trail while riding, it's less likely a safety switch issue and more likely a component failure like the CDI or stator. Check for any obvious melted wires or burned connectors near the engine.

I'd also check the pickup coil air gap. Some engines have an adjustable gap between the flywheel and the pickup. If it's too wide, the signal is too weak to trigger the CDI. Spec is usually 0.010-0.020 inches.

On my Suzuki King Quad, the pulse generator went bad. I tested it cold and it was fine, but after it warmed up it failed. Sometimes they fail intermittently. Try testing when the engine is hot.

Another tip: wiggle the wiring harness near the steering head while cranking. Lots of wires route through there and can get chafed. I had a bare wire grounding out the CDI signal.

Don't buy a new CDI until you've confirmed all the inputs are good. I wasted money on one once and the real issue was the stator. A service manual with testing specs is worth its weight in gold.

Check the engine stop relay if your ATV has one. It's a small relay that can stick open. Tap on it lightly while cranking to see if spark returns. If so, replace it.

I dealt with a no-spark on a 2005 Arctic Cat 400 and it turned out to be a bad connection inside the CDI harness. Re-pinning the connector solved it. Look for corroded pins.

I'd suggest checking the spark plug cap resistance too. Some caps have a built-in resistor that can go open circuit. Measure from the plug wire end to the metal clip inside the cap. Should be around 5k ohms.