Those are good numbers for a K&B 65. I have always liked the 12 x 8, 13 x 5 and 13 x 6 props on mine. I have had great results using stock carb. I never got ahold of a drone carb. I would love to play with one.
If this were an engine other than the "91" designation, it would have a better carb. These were specified by the military to run at a certain rpm (Randy at Mecoa wasnt sure what the spec was but guessed 5000rpm which is way lower than what its capable of obviously) on 12x6 wood props. Each engine came with a bag of 10 maple or beech props. I ended up with 3 bags of these props which I sold to an RCU member last year.
The problem with the drone carbs has something to do with the idle mixture disk. I think the "cats eye" is mis-machined or something. There was a rumor floating around awhile back about a batch of sportster engines with bad spraybars. Perhaps the military got a deal on a big lot of these engines with the fubar carburetors... They were bolted to a plane, fueled up, and shot in the air at WOT and then shot down. Pretty sad when you think about it...
They run out at WOT fine. The engine I ran today is the only one of my two I've ran. The other one is still un-run but has the same junky carburetor that this one has. I imagine with a good spraybar, the engine would perform properly. Idling for more than 5 seconds gives a very snotty midrange. Give it 10 seconds at idle (I could only get mine to idle down to about 4000 before they loaded up and quit.) and it'll quit on transition. I had the engine on an airboat I built but after having to retrieve mid-lake enough times, I pulled it off and stuck a Super Tigre 90 on the airboat. No more problems.
I will not even attempt to fly this engine until I get a carb for it. Do you have a spare carb that transitions well that you'd be willing to trade? I'd swap you my new un-used drone carb for a good one just for testing purposes on my test stand. The spraybar/mixture disk is roughly $14 from Mecoa if they even have any left for the non-RNV engines.
On my airboat, I got the fastest speed with the 13x6 APC prop - 30mph. The boat flipped when the bow came up a little and the wind caught it. Took me an hour to get the engine to fire again because I didnt have an electric starter. (I had a very sore arm after this adventure... but I was happy with the speed it gave) My Super Tigre S90K turns the 13x6 APC at 13,600 on my test stand but I haven't GPSed it since I got the 'Tigre.