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Rotax 912 power loss


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Dear Roger

I need some advice 

CTSW 2007 600H  The other day on a normal take of at about 300' the enjin loose power and run ,""feeling like a drop mag "

I was to high to land ahead and land on a crosss ground runway

Back on the ground the mags drop was 120 lt and rt with a 10 rpm diferance , run up smoothley

 We drain al the fuel and went through al the filters and lines  new fuel in

Next fligt no problems on ground run up but again full power 300' the same happen , ground run normal cowling of no abnormalitys obvious

The gay that servece the plaine came over and we took her for atest flight again the ruf run enjin  He  select lt and right mag with no chance

So it look to be a fuel problem ? Carb to be more presice ??



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Fuel problem is my first guess, but you already went through a bunch of of work already.


Have you performed a fuel flow test in accordance with the manual? After that, perform the dry restart test in the maintenance manual as well. These two things will eliminate fuel system as the problem if they pass.


Have you made sure your carb heat is functioning correctly? It's hard to see much of an effect by carb heat on the ground, but a takeoff run would certainly get temps up pretty quickly and start affecting your engine RPM.

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My experience would have you focus on the float bowls and the main jet. If the float bowls are clean, use a mirror and a bright light to look up into the main jet - a very tiny piece of rubber or other goop can cause your symptoms - which might clear up temporarily as it moves around or settles back into the bowl.


This is what may cause trouble in the float bowl:




And how little in the main jet can cause issues*:




In my case it was one time some silicone gasket maker that had come adrift. I've learned never to use silicone-based products around fuel.


This was the tiniest amount of gasket maker I had used to hold the float bowl gasket in place, that got all puffy and jelly-like from contact with gas:



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Thanks very much it really help


I will do the flow test and check the carb heat to be thorough and ,the  chap that service the plaine whant to bring his "wisper's carbs  and test fly it again to take it to his facility about 30knt away


Come to think of it the left  carb previously had a dislodge float ? the wire was out of place ? does it make sence ?

It is dificult to not be meckanical minded




Well at least i had good practise in power of emergecy landings , must say the last 2 landings a strong 20knt wind on the nose , and it help to have someone else in the plaine for the split desision if enough runway is left in front ,  Cut power 40 degrees flap  baloon as the attitude chance and power of landing in gusting wind  (1500meter runway so not that bad just not use to it )



Again  I realy  apreciate the help  and make me feel better that at least we are on the right track



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Hi Kiewiet,


Most likely a carb issue and many times you can not duplicate this on the ground verse the air. A quick easy way is to remove the cab bowl and look for debris and or a sunk float. If this fails then remove the carbs and completely clean them. 

Just a thought. 

If you are going to completely remove the carbs remember they are 7 years old and it might not hurt to just throw a rebuild kit in and be done with them for a long time to come. 

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I had very similar symptoms on my '06 CTSW. After weeks of work, which included full rebuilds of both carbs, replacing the fuel pump, checking fuel flow rates, etc., the trouble was finally traced to vapor lock. I had been using mogas, most likely winter blend which has higher vapor pressure.


I first suspected vapor lock after taxiing on a warm day, shutting down the engine, jumping out and opening the gascolator valve. Boiling fuel came *spraying* out (the initial boiling point is as low as 100 deg. F in mogas).


The clincher was hooking up a fuel pressure gauge and running up the engine on the ground. The fuel pressure fluctuates as vapor bubbles come and go...



Finally, I switched to 100LL (much lower vapor pressure) and have had *zero* engine trouble since (going on three years now since the switch). I'll take lead headaches any day over power loss in flight. :)


Edit: I, too, could never get the power loss to manifest on the ground... only in the air. I suspect the attitude difference (nose-up) might be the explanation. The fuel pressure fluctuation, however, was easily and repeatably reproducible on the ground.


Edit 2: Another possible explanation of in-air vs. on-the-ground differences: fuel flow into the carbs is higher in the air (specifically, at higher airspeed). This is because the prop "unloads" at higher airspeeds, the engine turns at higher RPM, and so it draws more fuel. This would tend to reduce the fuel system pressure further, leading to more vapor formation.

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Once of the guys in my EAA chapter has a Xenos motor glider, and when he first got it built he tried flying it on a hot day with regular pump gasoline.  He had a very similar power loss on climb out as what is described here, along with significant engine roughness.  He switched that airplane to 100LL and the problem was gone.


It seems to me that whether the airplane will have vapor issues depends on the amount of ethanol in the gasoline, and how well shielded the gasoline path is from excess heat that can boil off the ethanol (which happens at a pretty low temp - around 173°F / 78.4°C).  I have never heard of a CT having vapor issues, but if the ethanol content is high enough and it's a warm day and/or there is excess heat under the cowling for any reason, it's obviously possible.

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The mogas vapor pressure is an issue.  In California the vapor pressure is changed twice / year from summer to winter for easier starts and from winter to summer for better vapor lock protection.


Other states are different.  In states that have a year round pressure is it high for easy starting or low to avoid lock?

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I doubt ethanol content is the dominant factor. The vapor pressure boost from adding 5-10% ethanol is about 1psi, whereas the difference between summer and winter blend fuel can be as much as 7psi (going from 8-9psi for summer fuel to up to ~15psi for winter blend).






For reference, 100LL has a vapor pressure of 5.5-7.0psi.

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Surely if vapour lock is n problem , the enjin is certified for Mogas ? run for 7 years on it ?

There must be a way to rectify it (isolating lines / electric feul pump before the fire wall / a return valve ?)



I did a crude test of mixing water and fuel and see if the level chance after shaiking, we in SA still not sepose to have ethanol in the fuel


I learn a lot so far , Hopefully we will fit a diferent set of carbs as a test this week end and then take it from there


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Here is a interesting post on the question of mixing avgas and mogas and vapor point




""""""""Although I had run several tanks of 91UL mogas through my -6A during the winter months, I've been reluctant to do so during the summer months due to fear of vapor lock. While my plane was (and is still) in the paint shop, I thought this would be a good time to learn more about auto fuel in aircraft engines.


One aspect is detonation, and I'm still learning. Another aspect is vapor lock, and this is something that science can help solve. Pete Howell was good enough to stop by last week and demonstrate his simple rig for finding the pressure at which liquid would boil. One trip to Harbor Freight later, I had everything I needed. After talking with Pete and hearing about his and others experiences, I thought I'd test a range of mix ratios. I'd read on here that a small amount of 100ll would affect the vapor pressure of mogas disproportionately so I thought I'd check it out.


I sampled mixes of 1:1, 2:1, 4:1, and 8:1 mogas to avgas. South St. Paul sells both side by side so obtaining the samples was pretty trivial. All tests were done at an ambient temperature of 84ºF after letting the fuel adjust accordingly. Before starting, I measured 100LL boiling at 21" Hg of vacuum and the 91UL mogas boiling at 16" Hg vacuum at ambient temperature. The chart below captures my findings:

The actual pressure values aren't as important as the resulting curve. The curve clearly confirms that 100LL affects the vapor pressure disproportionally to the mixture ratio. With pure 100LL at 21" and pure 91UL at 16", I would have intuitively thought that a 1:1 mix would result in a boiling point of 18.5". The data shows however that a 1:1 mix has a pressure close to that of pure 100LL and that it takes an 8:1 mix to move the pressure to the halfway point between that of 100LL and 91UL.


I invite any comments, suggestions, or additional data on the topic. I think this is pretty cool stuff, and my thanks to Pete for the knowledge and techniques!


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This why I am adament to get the plaine to run on Mogas :


''''''It is possible but not recommended to use 100LL AVGAS, since the the lead content is like cholesterol to your engine: it will accelerate wear on the valve seats, create deposits in the combustion chamber and sediments in the lubrication system and gearbox. Increased maintenance is necessary to compensate. Unlike "conventional" aircraft engines, lead is absolutely not essential to the proper lubrication and operation of a Rotax 4-stroke aircraft engine. The increased octane rating also has no marked advantage for the operation of your engine.'''''''



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