the 3.6KY LP Onan genny's can be a real handful to diagnose with the symptoms you describe. After checking all the basics (compresion, ignition, fuel from tank to genny), you may want to replace the LP hoses on your set, due to their age. I have done this to several sets that were brought to my shop with this problem, and it seemed to fix issues. the fuel pressures to and from LP regulators are very critical, and any deterioration of hoses can cause problems. if you have any "backfiring or popping back" through carb., make sure to check rocker arms and valve lash.