i have had a similar problem with an older onan, when the starter button is held the ign power comes from the starter circuit, when its released it has to run off the generator, if the generator does not put out power it dies. could be a faulty controller board, fuses, mine was dirty commutator for the exciter from lack of use. i ended up jumpering the ign to 12 V constant so the motor would star running , and running a 100W light off the output to put a load on it till the slip rings cleaned them selves up, then hooked the ign back up as normal..