Re: frustrated mom
15 years? He's plenty old enough. Crash and burn on a 30 mph bicycle or a 60+ mph dirtbike? I think she made a solid choice.
Anyway, let's learn the basics of troubleshooting:
Fuel, air, compression, spark. If you have those, it should run (or try to run). Get them in the right proportions and it should run well.
Fuel: the carburetor controls that. There's usually an inlet valve that you can use to shut fuel off to it. There's a choke lever for cold starts (up is choke, down is for after it starts). There's a button on the left side for getting the fuel to flow in to the carb. I've never needed to use this. Ever. Carburetors don't just stop working suddenly unless something came off of it. I doubt this is the issue.
Air: there's an air filter. It's behind the "punisher" looking black plastic piece. If there's a plastic bag wrapped around the punisher, then it won't run. Otherwise, you probably have air flowing in. Getting air out is important too. If the exhaust got clogged, it could stop running. Ever heard of people putting potatoes in tail pipes? So the air part is probably not likely.
Compression: So the piston goes down, air and fuel go in. The piston goes up, and it compresses the air/fuel mix. Having compression in the engine is important. As was stated earlier, the compression can be lost (without warning) if the 4 nuts on top of the engine come loose. Tighten those bad boys down. Not too much, and do it evenly. If you crank them down too tight they can snap. You can tell if you have compression by riding the bike with the clutch lever pulled, then release the lever. Does it get hard to pedal? Hear a pop pop pop pop? That's compression!
Spark: This is where my money is. You need it to spark to work. When the piston comes up and squishes the air/fuel mix, the spark plug should fire at (or close to) the top. Then everything explodes and makes power! The piston goes down and the cycle repeats. So there's a magnet spinning on the right side of the engine. As it spins past a coil of wire, a little electrical signal is made (called induction) in the coil. That signal goes to a black box called the CDI. The CDI is where the magic happens. It stores electrical energy and fires a whole bunch all at once when the time is just right. Most times when the bike just suddenly quits, either the coil or the CDI is to blame. You can test spark pretty easily. Take the spark plug out, and put the wire on the spark plug again (so now you have a spark plug dangling by the wire) lay the spark plug's metal end on the engine (metal to metal contact is a must). Pick up the rear tire and pedal with the clutch lever released so the engine spins. See sparks? Good!
So there you go. Troubleshooting basics. Fuel, air, spark, compression. That's it. Now you know enough to fix it every time!