Onan Marquis Gold 5500 vapor LP generator

Thanks Barney...a bit of an update. There is an air relief tube on the side of the low pressure regulator. I blew it out with high pressure air and air seems to be moving freely through it. This did not solve the problem. It's still backfiring loudly through the carb. According to a lot of info on google, carb back firing is caused by a too lean air/fuel mixture which can be caused by a stuck intake valve or a engine that's needs timing adjustment. The strange thing is, this generator only has 31 hours on it. I'm guessing it wasn't exercised often enough and maybe there is corrosion on one of the valve stems causing it to stick. Does anyone have a suggestion on how to try to unstick the valve without taking the valve covers off? If I can't get it unstuck by spraying a product through the carb, then I guess I will have to take it in and pay a shop to fix it.