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Rotax 912 - interesting observation

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thilo.kind(at)gmx.net
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Posted: Sun May 17, 2009 9:31 am Post subject: Rotax 912 - interesting observation

Hi folks,



went to the airfield today for a short flight with my Zodiac CH 601 HDS with

Rotax 912 UL. During the pre-flight two of my flying buddies came up. One of

them started to turn the prop for the oil check. He remarked how 'easy' it

was to turn the prop. His plane is also equipped with a 912 UL, but the

compression stroke is much, much more pronounced on his airplane. We

actually did a side-by side comparison.



My engine (120 hours) has been like this since new. It runs fine; the Zodiac

is one of the faster ones in this class. I haven't checked the compression

yet, but oil consumption is very low (no need to re-fill between annuals).

Fuel consumption is also in the normal range. I believe the engine is just

fine. However, does anybody have an explanation for that?



Thanks



Thilo

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lucien

Joined: 03 Jun 2007
Posts: 721
Location: santa fe, NM

Posted: Sun May 17, 2009 9:46 am Post subject: Re: Rotax 912 - interesting observation

 	  thilo.kind(at)gmx.net wrote: 	
		  Hi folks,



went to the airfield today for a short flight with my Zodiac CH 601 HDS with

Rotax 912 UL. During the pre-flight two of my flying buddies came up. One of

them started to turn the prop for the oil check. He remarked how 'easy' it

was to turn the prop. His plane is also equipped with a 912 UL, but the

compression stroke is much, much more pronounced on his airplane. We

actually did a side-by side comparison.



My engine (120 hours) has been like this since new. It runs fine; the Zodiac

is one of the faster ones in this class. I haven't checked the compression

yet, but oil consumption is very low (no need to re-fill between annuals).

Fuel consumption is also in the normal range. I believe the engine is just

fine. However, does anybody have an explanation for that?



Thanks



Thilo 	




Does your friend have the 100 horse version? I.e. the 912ULS? That motor has a bigger bore and higher compression so it should be stiffer than the 912UL by a fair bit. 



Even so, on my 912ULS, one cylinder always feels really soft when cranking it over, but the compressions have always measured perfect and the motor runs fine. 



I'd do a leak down on it if you're concerned about it just to make sure everything's ok....



LS

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LS
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jimk

Joined: 30 May 2009
Posts: 2
Location: Illinois

Posted: Tue Jun 02, 2009 7:30 am Post subject: Re: Rotax 912 - interesting observation

I have a 912ULS with 25 hrs on it. I have noticed that if the engine is warm or has been run in the last 24 hrs the compression "bumps" are very obvious when burbing the engine. However, if the engine has not been run for a few days it is much easier to turn over and the compression spots are much less noticeable. 

 I had the Rotax 25hr check done by a Rotax Service Center but a compression check was not done and is not specified by Rotax until the  50hr check.

  Engine seems to be running OK.

  Still it makes me wonder if all is well.



Jim

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ptag.dev(at)tiscali.co.uk
Guest

Posted: Tue Jun 02, 2009 8:19 am Post subject: Rotax 912 - interesting observation

Hi! Jim

I don't think you have any problem.

Having stood for a period the piston rings and valve guides will have

lost the oil so you can expect compressions to be less sharp.

Regards

Bob Harrison G-PTAG (Europa c/w Rotax 914)



--

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jimk

Joined: 30 May 2009
Posts: 2
Location: Illinois

Posted: Tue Jun 02, 2009 9:35 am Post subject: Re: Rotax 912 - interesting observation

Thanks. I hadn't thought of that.



(Taking an 800 mile cross-country next week and was a little nervous about it)



Jim

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lucien

Joined: 03 Jun 2007
Posts: 721
Location: santa fe, NM

Posted: Tue Jun 02, 2009 10:37 am Post subject: Re: Rotax 912 - interesting observation

 	  jimk wrote: 	
		  Thanks. I hadn't thought of that.



(Taking an 800 mile cross-country next week and was a little nervous about it)



Jim 	




Mine 912uls is the same way. Super stiff when hot and looser by a bunch when cold. I've always figured that was because all the clearances are tighter when it's hot...



LS

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Thom Riddle

Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

Posted: Wed Jun 03, 2009 2:18 am Post subject: Re: Rotax 912 - interesting observation

I did the annual inspection on a Diamond Katana (912-F engine) a couple months ago. The prop gave very little compression "bump" resistance when cold. After warmup flight, I got 82/85 psi on all cylinders in compression check. This engine has about 1800 hours on it and still running strong.

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