I have 3 SK engines, 2 of the .91 and 1 of the 50 (actually .525). All run great. One thing you have to remember about SK engines is they are low rpm torque engines. The port size and timing is such that they will not tolerate a small prop and high rpm. My 2 .91s are real horses on a 15-6. Talk about torque. The 50 is good on a 12-5. Both have execllent idles and mid ranges. I recently has troughe with a Tower .75 running rich in the mid range so I dropped the SK 90 carb on it. Ran perfectly and flew great. Hobbico sent me a new carb and it runs fine, too. I think the first carb has a bad low speed needle. Anyway, the SK carb is excellent. Some of them can have burrs on the rotating barrel in the carb. One of mine had this and I touched it up with a fine file.

Oldbob who is running an 11-6 at 12,300 rpm, I would be surprised if you don't have leaning out problems in the air. No matter how rich you set the SK, the port size is not large enough to pass the fuel so it will normally go lean in the air and die. The 50 will turn a little faster than the 90, but at 12,000, you may be over the limit. Turk is running an 11-6 prop, but he runs no nitro fuel. I am running a 12-5, which is normally the smallest prop recommended. Don't tell me what the instructions say, they are incorrect, per the Kangke engineer. Please report back to us on how it flies. I would go to an 11-7 or a 12-5.