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Pulling Prop Through


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Is pulling the prop through a good indicator of compression?  Should it be about equal difficulty for each cylinder? 

I recently purchased my M20C and flew it home to PA from Texas with no issues.  About 3 weeks after having it cylinder 3 fouled the lower spark plug and I couldn't get it to check out during run-up.  I cleaned all of the lower plugs, reinstalled them, passed run-up and flew it with no issues.  The next time it wouldn't check out again...  I am under the assumption that I was not aggressively leaning it and just needed to the hang of it.  I decided to replace all lower sparks plugs because looking through the logs I noticed they haven't been replaced in many years (12 to be exact).  I replaced them with Tempest UREM37BY's.  After replacement I started it up and aggressively leaned, passed run-up, and took it for a flight with no issues.  All compressions were in the 70's in March when I picked it up but it is a high time engine (1800 SMOH).

The weather around here has sucked so I went to the airport just to put fuel in the tanks.  I decided to pull the prop through for fun and noticed that two cylinders are fairly hard to pull though, one is slightly less and another that is not very hard.

I just want to see if others think that the fouling of the cylinder 3 spark plug and having one cylinder with significantly less compression during a prop pull though could be related or if it should be checked further.

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Yes.  Pulling the prop through can catch low compression, but sometimes it's meaningless.

I had a plane a few years ago with chrome cylinders that seemed to have no compression cold, but great compressions hot.

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Is pulling the prop through a good indicator of compression?  Should it be about equal difficulty for each cylinder? 
I recently purchased my M20C and flew it home to PA from Texas with no issues.  About 3 weeks after having it cylinder 3 fouled the lower spark plug and I couldn't get it to check out during run-up.  I cleaned all of the lower plugs, reinstalled them, passed run-up and flew it with no issues.  The next time it wouldn't check out again...  I am under the assumption that I was not aggressively leaning it and just needed to the hang of it.  I decided to replace all lower sparks plugs because looking through the logs I noticed they haven't been replaced in many years (12 to be exact).  I replaced them with Tempest UREM37BY's.  After replacement I started it up and aggressively leaned, passed run-up, and took it for a flight with no issues.  All compressions were in the 70's in March when I picked it up but it is a high time engine (1800 SMOH).
The weather around here has sucked so I went to the airport just to put fuel in the tanks.  I decided to pull the prop through for fun and noticed that two cylinders are fairly hard to pull though, one is slightly less and another that is not very hard.
I just want to see if others think that the fouling of the cylinder 3 spark plug and having one cylinder with significantly less compression during a prop pull though could be related or if it should be checked further.


Pulling the prop through can indicate potential compression problems but I also have seen that the rings lined up perfectly enough to give you a poor compression and then it is fine the next time.

I would certainly do a compression test on it now and then again after it was run to verify it is still there.


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#3 cylinder fouled due to lead fouling or oil?  Lead fouling can be somewhat prevented with the AGGRESSIVE leaning on deck.  Oil fouling is a different problem.

What were the condition of the spark plugs when you pulled them out to replace them?  Clean and dry or oily?  Does your oil get black quickly after an oil change?

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33 minutes ago, Drumstick said:

#3 cylinder fouled due to lead fouling or oil?  Lead fouling can be somewhat prevented with the AGGRESSIVE leaning on deck.  Oil fouling is a different problem.

What were the condition of the spark plugs when you pulled them out to replace them?  Clean and dry or oily?  Does your oil get black quickly after an oil change?

I would say oily for cylinders 1 and 3 and mostly dry for 2 and 4.  The oil is fairly black now (about 20 hours since annual) but I haven't owned it for more than an oil change yet.  Oil consumption is about 1 quart every 6-7 hours. 

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2 hours ago, Kmac said:

I would say oily for cylinders 1 and 3 and mostly dry for 2 and 4.  The oil is fairly black now (about 20 hours since annual) but I haven't owned it for more than an oil change yet.  Oil consumption is about 1 quart every 6-7 hours. 

Oil fouling and black oil but good compressions is likely a worn ring issue.  On a high time C model its likely that it ran a bit "hot" over its 1800 hours but that is just guessing.  

I have an email from Mike Busch saying that this condition is not really a major issue other than its a pain in the butt and you always risk having plugs not clearing if they get oil soaked.

The lycoming run up procedure should be followed:  https://www.lycoming.com/sites/default/files/Magneto Drop-Off.pdf

Leaning the motor slightly during this run up seems to increase the EGT enough where it burns some of the oil off.

The fix for the oily plugs is to have new rings installed or new/overhauled cylinders.  Another symptom you may see is that you are blowing lots of oil out of the breather since you are pressurizing the case due to blow by.   You may also have a stuck ring or two and these can be freed with a solvent ring wash...your A&P should know how to do this as well.

I'm not an A&P.

 

 

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On 5/18/2018 at 4:01 PM, Drumstick said:

Oil fouling and black oil but good compressions is likely a worn ring issue.  On a high time C model its likely that it ran a bit "hot" over its 1800 hours but that is just guessing.  

I have an email from Mike Busch saying that this condition is not really a major issue other than its a pain in the butt and you always risk having plugs not clearing if they get oil soaked.

The lycoming run up procedure should be followed:  https://www.lycoming.com/sites/default/files/Magneto Drop-Off.pdf

Leaning the motor slightly during this run up seems to increase the EGT enough where it burns some of the oil off.

The fix for the oily plugs is to have new rings installed or new/overhauled cylinders.  Another symptom you may see is that you are blowing lots of oil out of the breather since you are pressurizing the case due to blow by.   You may also have a stuck ring or two and these can be freed with a solvent ring wash...your A&P should know how to do this as well.

I'm not an A&P.

 

 

Do you have info on the solvent ring wash?  I’ve never heard of it.

Clarence

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On 5/17/2018 at 3:42 PM, teejayevans said:


But they should be the same, yes?

AFAIK, the piston rings don't seal well when the engine is cold, so you may get wonky compressions if the engine isn't warm, and even then you still could have the piston rings lined up just right to let air through.

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On ‎5‎/‎20‎/‎2018 at 12:14 AM, jaylw314 said:

AFAIK, the piston rings don't seal well when the engine is cold, so you may get wonky compressions if the engine isn't warm, and even then you still could have the piston rings lined up just right to let air through.

Weather was beautiful last night (finally), so I took the plane up.  As soon as I shut it down I pulled the prop through again.  Each cylinder felt much more similar in compression and difficulty.  No issues with the run-up or plug fouling.  Thanks for the replies everyone.

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Weather was beautiful last night (finally), so I took the plane up.  As soon as I shut it down I pulled the prop through again.  Each cylinder felt much more similar in compression and difficulty.  No issues with the run-up or plug fouling.  Thanks for the replies everyone.


After a while you’ll get to know what is right and what isn’t. Sort of like when you have a baby. Some of that crying is for show, other times it means the diaper needs to be changed.
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1 hour ago, Marauder said:

After a while you’ll get to know what is right and what isn’t. Sort of like when you have a baby. Some of that crying is for show, other times it means the diaper needs to be changed.
 

 

 Here the new "diapers" cost at least $1100 each plus another 1000 to change one, and there is no hope of permanent potty training.  

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Just be thankful you don’t own a Continental engine, head cracking seems to be a major problem lately.  Then when you pull them you find stuck #2 compression rings.

Clarence

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