If it wants to run then stops suddenly when the starter is disengaged, check the ignition resistor. If the resistor is broken you only get spark when the resistor is bypassed in start mode. It will act as tho starting but will die when you stop cranking the starter.
I know this to happen with 12 volt systems. The resistor is used to cut down the voltage across the points so they do not burn out as quickly. On start up the full 12 volts goes through to give a hotter spark.
More info could help with the diagnosis. Good luck.
Rick
