Here's a simple trick to try when your Onan Generator QG 5500 EVAP won't start, or won't stay running. There is an altitude adjustment knob colored black with a blue dot in the middle. Normally, it is turned all the way to the left (0-2,500 feet), but you can adjust it by turning it to the right. All the way right is for 5,000 feet and up. If you are able to get your generator running, you can adjust this knob while running and listen to the engine until it sounds smooth.

Onan says that if you leave this knob turned all the way left at 0-2,500 feet, you will lose about 560 watts of power for every 4,000 feet of elevation you climb.