912 loses 500 rpm during climb
in 912 / 914 Technical Questions
7 years ago
The 912S is in a Phoenix, an LSA glider (it's a motoglider). The engine has 440 hours on it with no problems. I run it mostly on mogas, and it spends maybe third of it's flight hours with the engine idling while I'm soaring it. Here's the summary of what happened on two flights - Oct 7 & 8...
+ It started and ran normally on the ground, including the 3000 rpm ignition check
+ Full throttle while holding it with the brake yielded about 4800 static rpm (normal), then I began the takeoff.
+ Takeoff and climb were normal for the first 20 seconds or about 300+ feet AGL
+ I felt the engine running rough, and the RPM dropped to 4600 rpm
+ I reduced the throttle to 4400 rpm, where the rough running became smooth, and I could then power up to full throttle with no problems.
+ At 3000' AGL or so, I went to idle to begin soaring, and the engine performed flawlessly, as it always has.
+ There were other full throttle climbs during both flights, but at higher airspeeds, so the pitch angle was only 7 degrees instead of 12.
+ The Dynon Skyview data log showed the EGTs stayed with 20 deg F during the roughness, and with 30 deg F the whole flight
I thought that the nose up attitude during the climb after takeoff might have caused contaminants in the fuel to get picked up, but when I drained samples from each tank after the last flight it was pure mogas.
A friend wondered if it could the plugs, because I do a lot of soaring with the engine idling. A possibility? I've been doing the idling while soaring for the 3 years I've owned it, without any problems.
Any ideas about the cause? And what tests might help find it? At this point I don't mind doing another flight to carry out some tests to pin down the problem. With the 30:1 glide, I only need 200 feet AGL to turn back to the airport safely, so the risk period is about 10 seconds at most.
+ It started and ran normally on the ground, including the 3000 rpm ignition check
+ Full throttle while holding it with the brake yielded about 4800 static rpm (normal), then I began the takeoff.
+ Takeoff and climb were normal for the first 20 seconds or about 300+ feet AGL
+ I felt the engine running rough, and the RPM dropped to 4600 rpm
+ I reduced the throttle to 4400 rpm, where the rough running became smooth, and I could then power up to full throttle with no problems.
+ At 3000' AGL or so, I went to idle to begin soaring, and the engine performed flawlessly, as it always has.
+ There were other full throttle climbs during both flights, but at higher airspeeds, so the pitch angle was only 7 degrees instead of 12.
+ The Dynon Skyview data log showed the EGTs stayed with 20 deg F during the roughness, and with 30 deg F the whole flight
I thought that the nose up attitude during the climb after takeoff might have caused contaminants in the fuel to get picked up, but when I drained samples from each tank after the last flight it was pure mogas.
A friend wondered if it could the plugs, because I do a lot of soaring with the engine idling. A possibility? I've been doing the idling while soaring for the 3 years I've owned it, without any problems.
Any ideas about the cause? And what tests might help find it? At this point I don't mind doing another flight to carry out some tests to pin down the problem. With the 30:1 glide, I only need 200 feet AGL to turn back to the airport safely, so the risk period is about 10 seconds at most.
