| Air.
Fuel.
Compression.
Spark.
That's about the only things it possibly CAN be on these bikes.
Air: Dirty filter? As a test, try pulling the air filter off and putting a section of pantyhose over the mouth of the carburetor with a rubber band. Any air leaks around carburetor, exhaust port, reed cage, engine case? Spray these points with carburetor cleaner - any leaks will ignite. (Don't worry, it's safe - just shoot a tiny bit on there...)
Fuel: check your fuel lines? Carburetor? Fuel valve turned on? (I know, sounds like a stupid quiestion, but htere are people who really DON'T know about this ...) Dirty fuel filter? Washed out the tank with clean gas?
Compression: Are you getting any compression? Pull your spark plug, put your thumb over the spark plug hole, and pull the starter a couple of times. The compression should blow your thumb off the hole. If not, there's a leak.
Spark: Check the spark? Since you have the plug out already, put the wire back on it, and ground it against the frame or the cylinder. Pull the pullstart - what color is the spark? Blue-white, or orange? Blue-white is good, orange is okay, not great - means you may need a new coil, or at least an adjustment. Take the pullstart off - underneath is your coil and your flywheel. Your coil should be a business cadr's thickness away from the flywheel. If not, loosen the bolts, and readjust.
That's about all I can think of rght now ... good luck. | 
