So your engine isn't starting. You pedal up to speed, about 10mph, release the clutch and the engine turns over but isn't starting.
You need three things for the engine to run. Air, fuel and ignition.
How have you confirmed those three?
Are you using the choke? Is it open or closed? Lever up is closed or choke on, lever down is open, choke off. try leaving the lever at the 1/2 way point.
Have you confirmed that you have fuel flowing to the carburetor from the tank?
Have you removed the spark plug, held it against the cylinder head cooling fins and spun the rear wheel with the clutch engaged and observed a spark at the plug's electrodes?
Check these things and get back to us.
Tom