It's been a year and about 100 engine hours since I first reported this problem - see the thread
https://www.rotax-owner.com/en/rotax-forum/3-4-stroke-technical-questions/6353-912-loses-500-rpm-during-climb
During the year 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 a 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 two 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 only 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. The dealer has not heard of that problem, and thinks it is very unlikely.
The ground test runs with the added thermocouples on the front exhaust pipes had a strange behavior: At full throttle, the left TC reads about 800F, while the right TC reads about 600F. The rear EGT's read within 30-40F. Maybe the front TC's are not identical (look the same, ordered both at the same time)? Right cylinder running cooler than the left? A previous set of bead style TC's on the front exhaust pipes also showed the same difference, so maybe the right exhaust really is cooler.
Any suggestions, sensible and crazy, gladly accepted.
https://www.rotax-owner.com/en/rotax-forum/3-4-stroke-technical-questions/6353-912-loses-500-rpm-during-climb
During the year 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 a 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 two 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 only 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. The dealer has not heard of that problem, and thinks it is very unlikely.
The ground test runs with the added thermocouples on the front exhaust pipes had a strange behavior: At full throttle, the left TC reads about 800F, while the right TC reads about 600F. The rear EGT's read within 30-40F. Maybe the front TC's are not identical (look the same, ordered both at the same time)? Right cylinder running cooler than the left? A previous set of bead style TC's on the front exhaust pipes also showed the same difference, so maybe the right exhaust really is cooler.
Any suggestions, sensible and crazy, gladly accepted.
