As for the Onan statement...
I would guess your Onan is running on one cylinder due to a broken rod or the fly-wheel may be loose. A properly running Onan should be super smooth, way better than any Kohler ever was.
When the engine is running see if one or both exhaust pipes get hot. If one gets hot and the other does not then you'll know. Another way is to pull the spark plugs stick a wire through the hole and turn the engine over by hand and see if the wire moves.
Thanks. It is running on both as it idles real smooth but has nothing over half throttle. The exhaust gets real real hot and backfires when you shut it down. I idle it down as low as possible for a little bit.