Troubleshooting

Well, here's a quick lesson on 4 stroke engines and how they work and how to troubleshoot them. Keep in mind, whether it's a scooter, an atv, a mower or even a car, the theory remains the same.

First thing you need is:

INTAKE (The 1st Stroke) - During the intake stroke, the intake valve opens and the piston moves down drawing a charge of vaporized FUEL in to the cylinder from the carburetor.
COMPRESSION (The 2nd Stroke) - As the piston rises, the valves are forced shut by the increased cylinder pressure compressing the fuel/air mixture.
The POWER stroke - also known as COMBUSTION (The 3rd Stroke) - At the top of the compression stroke, the spark plug fires, igniting the compressed fuel. As the fuel burns it expands, driving the piston downward.
And finally...
The EXHAUST (The 4th Stroke) -  At the bottom of the power/combustion stroke, the exhaust valve is opened by the cam/lifter mechanism and the upward stroke of the piston drives the exhausted fuel out of the cylinder.

You can follow along with the diagram:
1=Intake, 2=compression, 3=Power/Combustion, 4=Exhaust

So, here's the quick rundown.
If you have fuel, check for spark.
If you have spark, check compression.

If you have fuel, spark and compression, it has to at least try to start.
If it has all 3 and doesn't even try to start, check electrical.