Chain of events leading to problem. Engine had been running fantastic and my normal 2 stroke oil (Penzoil aircooled) is now hard to find in this area so I just switched over to Stihl XP Ultra synthetic. I decided to take the engine apart and clean off the caked carbon from the top of piston and top of cylinder with my Dremel and soft brush. I've done this several times over the years with good results. Cleaned the engine up nicely, replaced carb and cylinder gasket and retweaked the needles. Engine was running great. The following day (last week) I flew the airplane 4 -12 minute flights and the engine performed flawlessly. Went out to fly yesterday and cranked up the Brison and noticed rpm was down a few 100 revs from normal at full throttle. It was quite hot and humid so I attributed that to a possible density altitude thing. Went to check throttle response (after a decent warmup) from idle to a quick 3/4 throttle and the engine gained a few hundred rpm, bogged down and stayed there and sounded kind of funny. Tried it again but advance the throttle slowly and it throttled up but still a few hundred rpm lower than normal. Opened up the low needle 1/10 th turn (higher than the norm) and that didn't help. Opened up the high needle a tad and really lost rpm. Pulled cowl off, removed and checked needles for debris and no issue there. Checked the little carb screen it was clean. Squirted fuel in and blew out all orifices. Then changed out my CH Ignition with a spare. An hour later I cranked her up again and it seemed good with decent transition and better top end so I decided to fly. Throttle up to full power and right at the end of takeoff run The engine lost quite a bit of power. I had to takeoff or crash into thick corn rows. Nursed it back to runway. Stopped the engine and it felt real hot. I let it cool for 15 minutes opened up the high needle some (higher than the norm) and started it up. Started right up and went to full power and in about 4 or 5 second at full throttle it started to sag and lost a quick 500 rpm. That was it. I packed it in for the day. So here is where I'm at now. Was thinking bearing problem because everything else appears Ok. Compression feels good and no noticeable bearing noise or grinding. Just remember Gary's dad saying to replace crank pin needle bearing after a crash or prop strike. He said it might be fine for awhile but eventually it will fail or maybe be on the outs to create an overheating issue??. The engine was in crash several years ago with same bearing. Almost sounds to me like I forgot to put oil in my mix but I did the night before @1.25 oz to 1/2 gal. of gas to give me 50:1. Geez Long story long! Truckracer, when I decarboned the piston and cylinder the ring seemed fine and was floating however I did not clean the ring groove. Could the oil change play a part in a stuck ring possibly turning the old congealed oil in the groove into JB Weld lol? Trying to fix the problem without sending it in if possible. Between jobs and money is tighter than you know what!!