I have some ideas and questions. If you could go back in time i would have made a temp fuel tank from a water bottle and some fuel line. Drill a hole in the cap slightly smaller than the tube and cut the bottom of the water bottle off. Use that to gravity feed brand new gasoline with a strong dose of carb cleaner. If the genny wont start remove the airbox cover and use starting fluid or brake clean. Just to see if you can make it fire up and cough out the badness. Once started the fresh fuel with carb cleaner might clear it out good enough to go camping. Will also show you if there is a fuel delivery issue.
I totally agree that removing the carb and using compressed air is ideal. Ive always wanted to invest in the little carb/jet cleaning pokie tools as well.
Im not very familiar with Onan carbs. Ive heard some people say you cant work on them. While others post about removing and cleaning them like any other small engine carb. Maybe differences in years?
Here are some questions that might help:
1. Does anyone have a link to the exploded view of his 1999 carb? I tried a quick search and found a carb but wasnt sure which model.
You mentioned there are only 2 adjustments. I wonder if the altitude adjustment screw uses a small oring and washer like you would find on say a fuel mixture screw. It seems like they are similar. If the oring deteriorated or got lost that could be why it runs bad. Or part of why it runs bad.
2. Since the engine runs decent with a load, shouldn't that point us closer towards the problem/s? The google image i saw showed a vacuum actuated choke. What if the vacuum is not working correctly with no load.....if thats even possible for some reason?
3. Have you tried running the genny and watched what the choke does during idle and then with a load? Maybe you could hold the choke wide open with your finger and see if the engine idles better after it warmed up?
4. Could also try swinging the altitude adjustment pretty far in either direction. If we think there is a slightly clogged carb passage, then it might run better if you adjust it for less air. Set it up at 8000' or whatever is highest. Thats not a fix. Just a way to get closer to finding the problem.
5. If you do the gravity feed gasoline method remember to have a catch gas for the trailer fuel line. Also without the cover on the genny it doesnt cool properly. So either run for short periods. Or setup a box fan. Just something to be aware for anyone that might read this.
Since you already removed it twice. Might be best to do it again. Not sure how easy it is to remove any plastic pieces but if you could soak the carb for days in cleaner and get a compressed air nozzle with the rubber tip you could blast out the passages pretty good. Or get a jet cleaning kit like someone mentioned above.
