912 non-compression before startup

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912 non-compression before startup

Postby stede52 » Fri Jul 18, 2008 11:34 pm

Has anyone out there experienced a situation with their 912 where you're getting ready to check the oil and when you turn the prop to burp the engine there is little or no compression for a while and after a number of turns and letting it sit the compression begins to come back?

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Postby Uwe Jens » Sat Jul 19, 2008 4:27 pm

Hi Steve,
I will ask some questions to see if I can help you, since what is happening with your engine is not normal.
How many hours does your engine have?
When you installed the engine, did you purge the oil system, and how did you do it?
Do you have installed a thermostast in your oil system?
Have you checked the magnetic plug?
In which part you operate and at what temperatures?  ( environmental)

One of the reasons for you not to have compression and it appears after some turns will have to do with the oil pressure, since it is the one that activates the hydraulic valves tappets for the correct function for the cylinder´s valves.

There is a bulletin, referring to problems that some engines have with the hydraulic valves tappets.  

You might have a problem with a seized valve and once you start to turn the engine the oil press increases, this lubricates the valve and it gets loose.

Have you noticed this for the first time or is always like this?

Help me with the information, so I can try to help you.

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Postby stede52 » Sun Jul 20, 2008 1:20 am

I just found out that my problem what due to sticky valves due to carbon build up and can happen due to the following:
It had been running and starting just fine until I started doing high speed taxing to get familiar with tail wheel.  I was running the engine at no more then 3000rpms during this time for a about 1.5 hours.  This is not good for the engine.  Today I went to the plane to try it again, three of the cylinders had compression, #4 didn't.  After a few minutes of warming up the engine and getting the temps up to operation levels #4 was still not firing.  I then shut off the engine for about 10 minutes and restarted.  All cylinder came up to temp just fine and it ran smooth as silk.  I then proceeded to run up the engine to 5200 for 15-20 to clean out carbon.  I'll see what happens tomorrow. According to a few sources this condition including Lockwood, will happen if the engine is run at consistent low rpms.  It likes 5000+ rpms and according to Lockwood Aviation you should be keeping the rpm above 5000 whenever you are cruising. I really didn't think it would carbon up that fast but I guess the high rpm engines need high rpms.......dah!~

I've got 15 hours on the engine
I do have an oil thermostat
the magnetic plug was clean
the piston heads showed no damage
the plugs were a mocha brown, just the way the should be
I'll take a look at the bulletin but I don't think that will be the problem based on the above

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Postby b1x4nqb » Sun Jul 20, 2008 8:01 am


I have a freind with the same engine and he also got 1 cold cylinder once in a while with his brand new motor.  It was shaking a bit as well for him.  He had no egt's, etc.  I'll bet it was also due to carbon buildup.  Sounds just like what you were discribing, from what I heard.  I'll pass that on.

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Postby scubarider2 » Sun Jul 20, 2008 8:30 pm

I heard that alittle sea foam in the gas and the oil along with running it at upper rpm's for a trip would clean things right up.  Also heard the mystery oil would do the same but some precautions with it.  Anyone else hear this?
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Postby alan » Sun Jul 20, 2008 11:20 pm

Ever since I was introduced to MMO by some ultralight guys I have used it. Used as directed (4oz for every 10gal of gas) it will prevent (not just reduce) carbon build up in a 2-stroke engine. It also drasticly reduces top end wear. MMO doesn't work quite as well as Avblend as an oil additive but it's a lot cheaper and works almost as well. Get it at Wally World for about 1/2 price in the gallon jug.

In order to actually clean out carbon build up, remove your air filter, tie the 'plane to a substantial tree, rev it up quite a bit, and lightly spray atomized water into the carb. The water flashes into steam during combustion and cracks the carbon loose. Your combustion chambers will look like new in short order. By atomized I mean from a trigger type application bottle on "spray". DO NOT "pour" water in.

In case you're worried about detonation or other harmful effects consider this: At Reno the Unlimited racers spray water into the intake of supercharged, highly stressed, WWII engines to stave off detonation.
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