I used to have one of these engines, it is a fine engine by all counts. However you need to take very good care for it when breaking inn. I used pennzoil at 32:1 for about 5 or 6 gallons. I sold before going to synthetic, but still fies in my club. It spins a mejzlik 22x8 at 7100 rpms and idles at 1500. It´s still on pennzoil.
I remember I tried a bambula 22x10 on this engine and the rpms droped a lot (about 6500) so I kept the 22x8. Some people here are getting between 6600 to 7100 on a variety of 23x8´s. Maybe your prop has something to do.
Some more info would help: How many fuel has been ran through it, what temperature is it reaching, type of muffler, etc that way you´ll get better help.

John