I have a five year old .91 that has only recently been run about 2 hours, been broken in properly and flown a dozen times in a Hogbipe with great success. It now starts easily, idles perfectly and runs well up until just about full throttle when it backfires and stops. Big change in operation with no root cause visible. On the rich side for high speed needle, no visible tank or line leaks and the valves appear to be properly adjusted. Some small resistance in crankshaft apparent when slowly turning by hand but assume that is cam operating. Very frustrating and hoping someone can help before I send the thing back for factory help.

....Bob....it's too lean. I just ran into the same thing....richen it up fully 1/4 of a turn
from where you think it is "good".

....you'll see what I mean.

FBD.

If your high speed needle is not responsive (You cannot make the mixture richer) then your low speed needle needs to be opened a little.

AND you may be over proping the motor.. What size prop are you running on the 91!!

LATER
Should be 14x6 or so..
May need to adjust the valves when you rebreak in!!

I bought a Saito 180 that had been only run very little then sat for a couple of years. It ran OK not much power on the high end. I seemed to get alittle resistance when I turned it by hand so I thought that maybe the bearings were shot. I sent it in to be looked, the diagnosis was that the old fuel hardened over time all around the bearing. When I got the motor back they returned the bad bearing and they looked like they were surround by sand. The motor ran great after that.

A backfire is usually a lean condition or possible sticky valve at higher operation. With that said the only resistance you should feel is the compression while turning the crank over by hand. I would dissemble the motor and crock pot it in antifreeze for that new "like" clean, IF the bearings are gooey and gritty like rcsport experienced it not to FAR of a leap for the valves to be gooey and sticky as well...............An engine that has set a while is a good candate for a cleaning to operate at top performance.

Just a thought.