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Old 12-18-2017, 06:50 PM
scottmillhouse's Avatar
scottmillhouse scottmillhouse is offline
Join Date: Apr 2007
Location: Madison, AL
Posts: 349
Default In flight Rotax power loss

First this is kind of long. Background: Rotax 912 ULS over 200 hours. A few hours after annual. On annual new plugs and the floats replaced on left carb. One float was heavy. Carbs were balanced before I found out float was heavy so they now appear to be a little off balance. Prop pitched a little coarser than before, testing the denser winter air with static around 5000 on ground and about 5100 in climb out. Flown about 5 hours in the latest configuration. Climb solo at around 1400 fpm today.

After take off climbed to 3000' and ran a few minutes at 5500 rpm at 124 KTS TAS. Slowed to 5350 for cruise and shortly after stabilized rpm suddenly dropped to 4700 and felt a little rough. Full throttle application caused no power increase. Reducing power the engine was much rougher. I turned around and limped along back home until the Dynon showed well within glide distance. I then stopped the prop and requested traffic clearance for a dead stick landing. In retrospect I should have kept the engine at WOT since it was relatively smooth there and not reduced power and shut it off until in the pattern but I was afraid whatever was wrong would be made worse. With prop stopped and holding 60 knots until touch down I averaged 600 fpm descent rate. I now know stopped engine glide performance.

On ground the engine started fine with normal engine run up. Powering up the engine felt starved and would not go over 4600 rpm and would shutter and run rougher the more throttle you added to increase rpm. Pulled plugs and if anything they looked lean with slight light gray deposits. Compression felt fine but I will run a compression check. Gasolator clear, everything in the green and as before except the in flight EGT difference.

Downloaded Dynon. It showed all in line up to about 1/2 way in my 5500 rpm run when the right and left EGT deviated from 712 and 700 to 726 and 793. That spread continued until I shut down the engine. Before I lost power fuel flow was 5.1 gph and it then deviated from 4.4 to 5.1.

I'm thinking a fuel line obstruction after the totalizer but before carbs so after the compression check I'll start disconnecting lines and checking fuel flow. Other idea is an ignition breakdown at rpm or under load. Hopefully nothing internal.


Before I start another s... storm on 5100 rpm in climb as I see being heavily discussed, that is at 75 kts climb which is only maintained for a short time. Cruise climb is well over 5200 rpm with rate of climb decreasing accordingly. Yes I'm pitched currently as a trial to exceed 120 kts but not at sea level or full gross weight but 3000' solo. It will even gain another knot or two when you run it up to red line.
Scott- 2021 donation
New RV-7A N579RV, just over 170 hours now
Built RV-12, 328 hours-sold, purchased RV-12 sold, Built RV-9A, 536 hours-sold, Not completed RV-7 sold, Built Kitfox sold

Last edited by scottmillhouse : 12-18-2017 at 07:15 PM. Reason: added text
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