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912 UL Compression Test

 
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hgmckay



Joined: 23 May 2006
Posts: 315

PostPosted: Mon Apr 06, 2009 5:34 pm    Post subject: 912 UL Compression Test Reply with quote

Today I changed the plugs on my 912UL (200 hours on the engine). I also did a compression check per the Rotax instructions Sec. 12 2.5. Here are the results of the compression test:

Cylinder 1 Gauge 1Test Pressure 87 psi Gauge 2 pressure 86 psi 1.15% Drop
Cylinder 2 Gauge 1Test Pressure 87 psi Gauge 2 pressure 76 psi   12.64% Drop
Cylinder 3 Gauge 1Test Pressure 87 psi Gauge 2 pressure 86 psi   1.15% Drop
Cylinder 4 Gauge 1Test Pressure 87 psi Gauge 2 pressure 86 psi   1.15% Drop

Every thing looks fine except cylinder 2. The percent pressure drop is still within the Rotax allowable drop of 25%, but because there is such a large difference between #2 Cylinder and the other three I am concerned. I repeated the test on cylinder #2 twice and got the same result. During the test on number 2 cylinder I also heard air bubbling back through the oil tank. This seems to tell me that one of the valves was partially open allowing air to pass out of the cylinder. I assumed that I was on top dead center because the prop did not move when the pressure was applied. Could I be slightly off dead center with one of the valves cracked open and the prop not rotate? I need some advice and guidance as to what I need to do to find out what the problem is, and how to correct it when I confirm what is causing the 12.64% drop in pressure in cylinder #2.

Hugh McKay
Allegro 2000
Rotax 912UL (200 Hrs.)

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Float Flyr



Joined: 19 Jul 2006
Posts: 2700
Location: Campbellton, Newfoundland

PostPosted: Mon Apr 06, 2009 7:20 pm    Post subject: 912 UL Compression Test Reply with quote

Hugh:

Interesting that most compression tests are done with 80 PSI feed. I’ll keep that in mind for the Rotax.

If you were hearing air in the oil tank then that indicates air passing a compression ring into the base of the engine then back up the scavenger line to the oil tank. Sounds like you may have a little something under the #2 ring. You may want to try a little run with some Sea Foam or other cleaner in the fuel then test again. This may be a good place to run a little E -10 through it on the ground just before an oil change. Keep an eye on it and if the compression starts dropping faster, then you may want to have a closer look at that cylinder.


Noel

From: owner-rotaxengines-list-server(at)matronics.com [mailto:owner-rotaxengines-list-server(at)matronics.com] On Behalf Of Hugh
Sent: Monday, April 06, 2009 11:00 PM
To: 'Rotax Engines'
Subject: 912 UL Compression Test



Today I changed the plugs on my 912UL (200 hours on the engine). I also did a compression check per the Rotax instructions Sec. 12 2.5. Here are the results of the compression test:

Cylinder 1 Gauge 1Test Pressure 87 psi Gauge 2 pressure 86 psi 1.15% Drop
Cylinder 2 Gauge 1Test Pressure 87 psi Gauge 2 pressure 76 psi   12.64% Drop
Cylinder 3 Gauge 1Test Pressure 87 psi Gauge 2 pressure 86 psi   1.15% Drop
Cylinder 4 Gauge 1Test Pressure 87 psi Gauge 2 pressure 86 psi   1.15% Drop

Every thing looks fine except cylinder 2. The percent pressure drop is still within the Rotax allowable drop of 25%, but because there is such a large difference between #2 Cylinder and the other three I am concerned. I repeated the test on cylinder #2 twice and got the same result. During the test on number 2 cylinder I also heard air bubbling back through the oil tank. This seems to tell me that one of the valves was partially open allowing air to pass out of the cylinder. I assumed that I was on top dead center because the prop did not move when the pressure was applied. Could I be slightly off dead center with one of the valves cracked open and the prop not rotate? I need some advice and guidance as to what I need to do to find out what the problem is, and how to correct it when I confirm what is causing the 12.64% drop in pressure in cylinder #2.

Hugh McKay
Allegro 2000
Rotax 912UL (200 Hrs.)
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Roger Lee



Joined: 10 Jan 2006
Posts: 1383
Location: Tucson, Az.

PostPosted: Mon Apr 06, 2009 7:40 pm    Post subject: Re: 912 UL Compression Test Reply with quote

Hi Guys,

First thing is Rotax leak down compression test is done at 87 psi and not the standard 80 psi like Lycoming's and Continental's. Run your engine once and do the test on number two cylinder again, I will bet it goes up to the other ones. I have had this happen once or twice and a run with a retest and it was fine. The engine also need to be up to temp. So I do a carb balance on it as one of my last issues on an inspection and then the compression test after I shut down. Do not do a compression test on a cold Rotax. You should not hear any leaking air in the exhaust or the intake, but you will absolutely hear a little in the oil tank and that is normal. Where do you think the extra 2-3 psi of air goes on the leak down. Normally I see 87 over 85-84 for normal test pressures. Here is a picture of a little test tool I made for compression test. Easy and cheap to make.
And while I'm thinking of it you do not use any antisieze on the spark plug threads. It is mandated in the Rotax manual to use a heat conducting paste. You can pick some up from Lockwood Aviation in a little clear packet or a larger tube. One little packet will do one plug change. Do not get any paste near the plug electrode. Keep the paste up at least 4 threads. It will cause misfires, it is a silicone base.


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Compression Tester End.pdf
 Description:
Compression test tool end- A spark plug

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Compression Tester.pdf
 Description:
Compression test tool

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Dick Maddux



Joined: 24 Jun 2008
Posts: 496
Location: Milton, Fl

PostPosted: Tue Apr 07, 2009 3:52 am    Post subject: 912 UL Compression Test Reply with quote

Roger,
Would you concider making one of those Rotax compression testers for me. I have one for a Cont/Lyc and it was a handy tool but of course it won't fit the Rotax. I will pay you what you want.
  Thanks,
      Dick Maddux
      Rotax 912 UL
      Pensacola,Fl
Feeling the pinch at the grocery store? Make dinner for $10 or less.
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Thom Riddle



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

PostPosted: Tue Apr 07, 2009 4:16 am    Post subject: Re: 912 UL Compression Test Reply with quote

Hugh,

First - If you are not right on top dead center, the prop will definitely spin.

Ditto what Roger said with one minor difference. We usually do a compression test during first part of inspection while the Rotax engine is cold and usually get almost the same readings as when fully warmed up, generally no more than 1-2 psi difference. This reading is not precisely valid but it will show up major problems early in the inspection and save us time and the customer money by not wasting time on minor issues first. For example, you don't change the oil/filter, plugs and do a carb balance on an engine that needs a valve job. If there is a cylinder that is much lower than the others but still within service range, then we'll repeat the compression test after full engine warm up later in the inspection. If they are all reading good cold, you accomplish little by doing it again hot. We always note in the log book that the compression numbers were taken either cold or hot, whichever is the case.


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daveaustin2(at)primus.ca
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PostPosted: Tue Apr 07, 2009 4:48 am    Post subject: 912 UL Compression Test Reply with quote

I'd suggest trying the use of Marvel Mystery Oil before you do anything radical. It unsticks valves and rings etc.
Worth a try.
Dave Austin 601HDS - 912
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Goodone



Joined: 13 Sep 2008
Posts: 49

PostPosted: Wed Apr 08, 2009 1:09 pm    Post subject: Re: 912 UL Compression Test Reply with quote

I have had wonderful results with Marvel Mystery Oil in my cars over the years when placed as directed in either my car's oil or gas. The most dramatic success in recent years was putting it in the engine oil to successfully remove a truly bad, bad knock in the engine of my 97 Olds Bravada. However, I have not added it to the oil in my 912 because of fear it might cause foaming in the gear reduction unit of the engine and result in damage. If folks have used it in the oil of the 912's with good results I would like to hear about it.

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