RE: Welcome to Club SAITO !
Dancer:
Prop strikes often damage the bearings, you might not feel any roughness but the races could be brinnelled and the balls have flat spots. If this is the case they will take more power to spin the shaft, lowering your peak rpm. A few more hours, if your bearings are damaged, will see then howling. A new set of composite stainless/ceramic bearings from RC_Bearings.com will run about $30-35, I'd change them.
With 3 gallons through the engine you probably have 7-10 hours on it, and while that's a little low for the stock bearings to fail (without the dirt nap) running all synthetic oil will in itself shorten their life. And you're going to want some good bearings before long anyway.
I did one engine a while back that seemed to have good bearings, but I couldn't get the rpm up on that one until I changed the bearings. Just as a guess you have the same thing.
Nothing wrong with the YS 20/20 fuel, so long as you add four ounces of good castor oil to each gallon.
Bill.