10. Put the plug back in the cap, then ground the threads of the plug to a metal part of the engine. Note that Alex has the plug touching the bolt for the cam cover. The cover itself has a rubber gasket and is painted, so it wouldn't provide good ground. Kick the engine over or hit the starter button and look for spark. Even in sunlight you should be able to see spark between the tip of the plug and the electrode.
11. Check the integrity of every electrical connector on the machine (a dirt bike doesn't have that many). The wires plugged onto this Honda coil are easy to knock loose when removing and replacing the fuel tank. As you inspect connections, look for obvious cuts, tears or burns in the wires between the connections. We've seen very small cuts to wire insulation that have severed the wire inside.
12. When there's no spark, a very common culprit is the kill button. It's easy to unplug the kill button from the system. That will usually defeat the kill function. If the button or its wiring is at fault, disconnecting it will generally restore spark. On some models of kill button, parts can easily fall out when the button is off the bar. So if you changed the bar and then the bike wouldn't start, replace the button.
13. If you've chased the spark this far and haven't made any progress, hopefully you're still in the garage. You won't be riding today or if you're at the trail, any farther today. A shop manual has procedures to test electrical components like this stator, but the most surefire method is to swap ignition parts with a bike that runs until you do have spark. Most of the time, the stator will be the culprit. This one looks fine but is bad. Stators will often work when cold and go bad when hot, making them difficult to diagnose.
14. If you do locate spark and fuel is in all the appropriate places, the problem is mechanical. You can use a compression gauge on an electric-start bike, but it's difficult at best on any kickstart bike. The best option for a four-stroke is a leak-down test. This leak-down tester is from Motion Pro and it will tell you whether the engine is holding pressure and is a great indicator of engine condition.
15. If the engine isn't holding pressure for the amount of time specified in the leak-down tester directions, remove the muffler and the airboot. Start the leak-down test again, then listen for air escaping from the exhaust port (bad exhaust valves or seals), carburetor intake (bad intake valves or seals) or along the head-to-cylinder joint (bad head gasket or warped head). A cardboard tube from inside wrapping paper or paper towels can be used like a stethoscope to help locate the compression loss. If it's the rings that are leaking, you will hear air escaping from the crankcase vent hose.
At this point you will have exhausted the options open to most home mechanics. Generally you will have located the problem. If you haven't, from here you will need to start looking inside the engine to make sure all is correct. But make sure you have covered all the bases. If you feel that the problem is still something small and external to the engine, you could save in the long run by having a good tech troubleshoot the bike before taking it apart. There is a good chance that a tear-down will obscure what caused the original problem, so make sure that all other avenues are explored first.