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In flight Rotax power loss

scottmillhouse

Well Known Member
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.

Ideas?

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.
 
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Could be but unlikely in a carb since I have two carbs and both sides appear to have lost power at the same time. (my logic was wrong, only the left carb was effected and that pulled the right side down and total power) At annual I replaced gasolator screen only because I damaged it upon removal for inspection. Zero deposits on screen or in bowl with 200 hours and two years.
 
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what's the fuel pressure at WOT ? If you aren't showing low then I would check carbs

are you getting full unobstructed range of movement on the throttle on both carbs ?

I would check there is no fuel leaking out of the bodies and do a reset of mechanical and pneumatic sync before anything else

any cracks in the rubber throat attachment to the inlet manifold ?
 
Have you pulled the cowling yet? I had a carburetor work it?s way loose about 5 Hours after reinstalling when it didn?t seat properly in the socket. I had some similar symptoms.
 
answers

Fuel pressure over 5 psi, normal range, full throttle travel, choke not on, did remove cowl and all looked good including carb sockets, no fuel dripping or anything looking out of place. 93 octane auto fuel with ethanol about 3 weeks old, plane was not flown for two weeks, OAT about 50-55 on ground. 4 degree dew point spread but had same problem an hour later after under cowl inspection so not carb ice.
 
Those are classic symptoms.
Check the obvious first.
Drop the carb bowls and check for debris at the bottom of the bowls. As you remove each bowl (instructional video on uTube) be careful not to spill/empty out the bowls, so you get a good read, on any foreign objects, submerged or floating around.
I bet debris is clogging one or both of the jets.
It only takes a tiny speck, to cause a rough running engine.
 
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Could be but unlikely in a carb since I have two carbs and both sides appear to have lost power at the same time...........

I can?t quite figure out the above conclusion, since you reported in your original post quite a large difference in EGT berween sides when the loss of power occurred. That indicated to me a difference in mixture between sides, and therefore a difference in power generated, which would explain the roughness. IMHO it is likely one of your carbs has a blockage.
 
In addition to a trash check in the float bowls, note that Bings can be blocked with a flake of trash in the fuel inlet, upstream of the float needle.
 
In addition to a trash check in the float bowls, note that Bings can be blocked with a flake of trash in the fuel inlet, upstream of the float needle.

I have seen more than one Rotax 912 exhibit these symptoms and it is usually trash in the carb bowls that is blocking the jet, or as Dan says.

Vic
 
I just went through this dance...except I was losing fuel psi. Found a ball of shredded plastic shavings in fuel line fitting coming out of engine fuel pump.

With your pressure reading normal; i?d pull the individual fuel lines to each carb and inspect (along with carb obstructions within the carb)..

The material looked like the inside of the fuel hose...could have been caused when fittings were installed as it was nothing that could have made it through the gasolator.
 
Besides the obvious debris in the carb, check the balance tube between the two carbs. I have seen blockage/crimped as well as worn through to a hole.

Next big one is ignition, did you do a mag check, or an ignition pack check. One could be faulty or just loose wiring to the pack. This is where 4 x EGT probes is more diagnostic than the two collector averages.
 
Could be fixed

Thanks for everyone's input. It kept me focused to carb obstructions. Went back out today. Ran compression all great. Pulled both carbs. Zero trash in bowls, no jet restrictions. Disconnected lines and flushed, reconnected and flushed through carb. Reassembled and engine runs fine. Once warm ground static up to 5000 rpm like before. Did not see any trash but it is possible that something got dislodged when flushing. Did not balance carbs or fly due to rain. Will repeat and rebalance carbs, install cowl and do some high airport circuits to test before I trust it. Wish there was visible cause so I know it is fixed.

To complete the post. I ran the engine up twice with all ok and then reassembled to fly. Once again power reduction. With our weather it sat again for about another 2 weeks. I disassembled everything, flushed and removed all auto fuel, filled tank with 100 LL and flushed again. Reassembled and flew today. A few circuits high above my airport and then at altitude in area. All worked great with EGTs matching. I think this was a fuel issue. Filled for a Xcountry that never happened and it aged with wide temperature swings with only a couple of short flights.

Update: After flying the last couple of months with 100LL, absolutely no problem. Assumed alcohol in aged auto fuel created a blockage in lines. Will go back to auto fuel when weather turns and I can fly at least weekly. I learned my lesson and will switch to 100LL for winter from now on.
 
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I experienced the same couple of months ago. Just before it I replaced fuel lines and particle ended to carb bowl.

As an end result of that I ended up to overhauling carbs and flushing fuel system. Like said, it takes tiny flake to restrict fuel flow and engine is shaking due to unbalanced fuel flow to both sides of the engine.

In my case it took couple of hours to build trust back to the engine.

Have any of you converted your fuel lines to braided stainless steel PTFE fuel lines? With Teflon lines you don't have to worry ethanol and replacing those all the time.
 
I experienced the same couple of months ago. Just before it I replaced fuel lines and particle ended to carb bowl.

As an end result of that I ended up to overhauling carbs and flushing fuel system. Like said, it takes tiny flake to restrict fuel flow and engine is shaking due to unbalanced fuel flow to both sides of the engine.

In my case it took couple of hours to build trust back to the engine.

Have any of you converted your fuel lines to braided stainless steel PTFE fuel lines? With Teflon lines you don't have to worry ethanol and replacing those all the time.

After finding the ball of shredded plastic looking stuff that looked like the inside of the fuel lines that came with engine...I just eplaced with the kit that Aircraft Specialty sells (really nice setup (all Teflon/Braided SS w/AN fittings)..will not have to worry about 5 year change-out.
 
Ric and All,

The RV-12 Conductive Teflon fuel line retrofit kit is currently on between 60-70 RV-12 aircraft as well as a variety of other 912 powered aircraft.

It was originally designed as a replacement to the rubber hoses that were being supplied with the engines. We had many customers call and ask us to design a system that used conductive Teflon instead of rubber. It provided a cost effective alternative at the 5 year replacement mark for a hose package that did not need to be changed out at 5 year intervals.

Ric's old hoses are currently enroute to us and we will dissect them once they arrive and see what we find.

Steve

P.S. As a side note, we are still collecting data, but we have not had any complaints (utilizing our fuel system) of the sporadic low fuel pressure indications that seem to affect some of the rv-12 utilizing the stock fuel system.
 
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Similar Power Loss

I posted this several months ago relative to a similar power loss:


Part of my power loss was attributed to sticking carburetor pistons. As I understand it, the pistons move to control carburetor inlet air as the diaphragm moves in response to air pressure changes. The piston motion can be checked by removing the air filter and sticking your finger in the inlet and lifting the piston up and down. The pistons should move freely with very little force. The carburetor air vent lines should end in the same relative location for each carburetor so that each carburetor is sensing the same ambient pressure is another suggestion. Not sure of all the things that can cause sticking pistons but excessive oil on the air filter and bad (old) auto fuel are examples I am aware of. I had EGT differences as high as 200 degrees and reduced power (4,300 rpm) during my episode.

I sent my carbs to Lockwood for the 200 hr service and haven't had any further problems. I also started adding Stabil to my premium auto fuel since I had the power loss problem.

Bob Kibby N712BK (371hrs)
 
Bob,

Excellent point! Make sure that those short tubes are not exposed to slip stream at all and generating suction to carb altitude balancing. I experienced that during the maiden flight, unfortunately. I let tower to know that I have some engine problems. They cleared me to land any of the 4 runways or grass as long I land safe. I made back to airport. Solved problem by replacing extended hoses by original tubes.

Hope this is on the Lessons Learned list.
 
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