It's been a year and about 100 engine hours since I first reported this problem, during which the engine performed flawlessly - until a week ago. The same symptoms occurred right after takeoff, and I shut the throttle, easily landing on the remaining runway. During ground test the next day, the 500 rpm drop occurred; a mag check while it was running at 4500 (instead of 5000) showed normal (~100) rpm drops. The fuel is clean, the carb bowls are clean, the floats all weigh the same 3.1 grams, and I didn't find anything abnormal.
I bought some type K thermocouples which I strapped to the each of the front cylinder exhaust pipes, and ran the wires to a dual channel thermocouple meter in the cockpit. Several ground runs later, the rpm drop had not appeared. I and a very experienced 912 owner/mechanic who is helping me are very puzzled. His best guess is a valve in one of the front cylinder heads may be sticking, but since those cylinders don't have EGTs, it's difficult to say that happened.
Another 912 pilot suggested the two position propeller (1-"under power" and 2-feathered for gliding, actuated by a cockpit lever that pulls a cable) might be moving to a coarser pitch, which would slow down the engine and could produce the slight rough running I noticed. I've contacted the dealer, but not heard back yet.
Any suggestions, sensible and crazy, gladly accepted.
