A lot of them have limiter on them for emissions rules, and when they come back from Onan for work, the plastic know is always gone because they take it and it's limiter off to get more range. The altitude adjustment is really just a mixture adjustment, so it can be very useful if you are working on cleaning out with Seafoam, as you can adjust the mixture to run better as it cleans out.
If you look at your manual, or one on line, it will show which way to turn the adjuster for higher of lower altitude. You would want to turn it in the lower altitude direction to compensate for plugging and surging. We had it to the full low altitude, richest setting, beyond the limiter, when we were cleaning ours out, as it was the only place it would run very well.