Just a guess but I'm thinking your tach may be off by about 1000rpm. I still use a couple of the 91s and mine run about 8900 and idle at just under 2000 with the same prop. That's off the top of my head though, haven't had the tach on them for a long while, doesn't mater though, it is what it is and it's hard to over rev the engine with that prop. As long as they fly the plane like you want it and it stays in it's working RPM range your OK. I found the engines ran great with the 14 inch prop but I like my planes to fly slower with more torque and the 15X4 gives me that. I have a three way switch on my radio I use for the throttle, high rpm for starting and warming it up, down a click for the low idle, mine idle at about 2000K or a bit lower for idling out to the flight linewith just adding a couple of clicks of throttle, as soon as it's in the air I click it back onto the high idle so I don't get any flam out during stunting. Low idle to land and the last click of the switch is to kill the engine. Works well for me but I have been doing it that way for a lot of years and it all happens naturally without thought.
Ask me sometime why I started rebuilding all my own engines and never send them in for service.