First question, have you had a lean run at all? Blown a plug? Sometimes when these engines get hot, the head bolts will loosen up just enough to let the head plug turn when you tighten down the glow plug. That in turn, turns the sleeve and then it doesn't line up properly with the ports. The easiest way to check it is to remove the muffler and look into exhaust port. The opening in the sleeve should be dead center in the hole. If it isn't, you will have reduced performance. That may explain your low rpm readings.

Hope this helps.

Blessings, Terry