Sometimes it can spark outside the cylinder but not spark enough under compression. Clean your ground wires and check the plug boot for tightness.
You might just be flooding it. Is the plug wet? Let it dry out and try again. Are you sure it’s getting gas. Pour a little in the plug hole and see if it fires. Could be broken reed, dirty carb, fuel line air leak or popped out crank seal.
When you prop it through by hand does it have allot of resistance? Might have stuck a ring last time you ran it.
FAA certified jet engine and aircraft technician,
Nicholson Speedway class champion 2001,
Yamaha KT100 Service Center,
41 years karting experience