Sounds like you need to rebuild the carb...Its gonna cost you like $100 for the rebuild kit, but it will work GREAT!!! mine run beautifully now..Onans are real power houses..they will sit and run all day and give you great power...and some are economical..My 16hp engines are ok, but my 18hp and 19.9 are gas hogs!!
If it runs a lot differently without the air cleaner than with it, ther's your problem. Too much restriction in the air cleaner. You have either a dirty element or some other restriction.97XG