One cylinder low on compression

[aerhed]

I am afraid I have never understood mechanics who want to pronounce a problem after doing a COLD compression check. Yeah, it's easier to do the compression check when the engine isn't going to burn your hands, but the purpose is to get a good compression check, not make it easier to do the test.

Paul, after only 30+ years of doing compression checks this is what I've learned: A good jug is a good jug. Hot, cold, don' matter. Sure they'll often read better hot, big deal. If they don't check pretty good cold, there's something wrong inside. Not always broken, but something wrong. A good jug will only gain a couple pounds hot vs. cold. Many times carbon or lead is the culprit, but often this is just an assumption reinforced by a false fix. Typically someone will stake the valves on a leaker and get a better seal. Then they'll blame it on carbon/lead when in reality what they've done is just knock a poor fitting valve face into it's sweet spot. Voila, she is fix! Not so. People argue that all the parts don't fit right if you're not at operating temperature. Not correct. Maybe true, but not correct. They fit cold when they were new, why not now? Also, many ring problems are masked by excessive oil in the jug. Many oil pumpers will show mid-seventies on compression only because the rings are coked & oily. Were you to clean the rings & cylinder walls you'd find that it actually sucks. So, checking hot really only helps to fine tune a trend analysis. Even then there is a lot of variability between compression checks because rings move around even if the valves are perfect, which they often aren't. Summary: Good is good, bad is bad, and it's not that big a deal to rip off a jug & fix one. That's one big advantage of an opposed engine with bolt-on jugs, easy to fix.

[Ironflight]

Aerhed,

I really don't disagree with you - a truly bad jug is going to point itself out either way. I get annoyed when you find one that is marginal, kind of on the edge, and the mechanic goes "if it were just a couple psi better, I'd pass it..." Well, you can get a couple of psi between hot and cold. But all the really bad jugs I've seen could be found by just pulling the prop through - the bad ones are noticeable by feel. But like you said, if you want to do consistent trend analysis, doing it the same way every time is important. You probably know a bad jug is bad before you pull out the compression tester.

Paul

[AlexPeterson]

My engine has consistently (for 1250+ hours) had compressions about 78,78,76,72. Each annual is more or less those numbers. I have always been able to hear air blowing by the rings on #4 if I listen in the oil fill hole.

Even with the lowest being as high as 72, if I pull the prop through four cycles, I can tell you which one is #4. If you (check ignition!) pull your prop through, and one is really at 50, it will REALLY be noticeable. I'd fly it and pull the prop through after the next several flights, and see what you feel. If something really did change, you'll feel the soft cylinder and can investigate further.

[Adam]

Iv had a similar issue and found that air was leaking into exhaust stack, what we found was a sticky valve. We drop the valve, honed it out and put it back in service, flys fine and compression is fine..

[gereed75]

I also make it a habit to do the poor man's comp test prior to each flight. I also have one cylinder that seems a bit softer than the others.

It seems that if I go back and check it the second time, it seems "up to snuff".

IO 360 with chrome cylinders that runs great but burns a bit more oil than I'd like (1 qt per 15 hours).

Normally have my A/P do the comp check at condition check, but I'll do the next one and pay closer attention after reading this good info. Thanks

Fair winds

[Darren S]

Ok, I just wanted to update my original post. I spent a good half day at the airport with the fella doing my engine work.

From my reading last night, talking to some people and the responses to this post, I figured my first chore was to "Stake" the valve and then check the compression again. Run the engine, warm it up and check it again.

So we took off the rocker cover of #3, got a block of wood and gave the rocker arm of the intake and exhaust valves a good couple of whacks !! Very scientific

BTW, the air leak was from the intake valve as you could hear air leaking from the inlet.

Check of the compression yielded a compression of 68/80 !! Big jump from 50/80. So next we ran the engine, warmed it up and the compression came up to 75/80 !! I was all smiles.

There were some decent sized lead chunks on the plugs of #3 and #4. So I pose the question of, "What can I do to help prevent lead build up on the valve faces ?"

As stated earlier, I lean at idle, lean at taxi, lean in my run up, lean in flight. In cruise I lean to peak and then run 25 - 40* ROP. Evidently #3 is the cylinder that peaks first.

So thanks to all for chiming in and tell me what you do to help prevent lead deposits, or is it inevitable?

Darren

[Jeff Vaughan]

http://www.decalinchemicals.com/

So far no sign of lead deposits. 195 hours

[dtw_rv6]

Originally Posted by Darren S

So thanks to all for chiming in and tell me what you do to help prevent lead deposits, or is it inevitable?

Darren

Your first post said you are running a Superior engine. I've been running unleaded gas in my Superior for 4 years -- occasionally 100LL when I'm travelling. No lead problems anywhere.

Don

[Darren S]

Dang Jeff !! Thanks for the link. Never heard of the stuff but will certainly give it a go. This lead build up issue gave me a scare.

Any negatives with using TCP ?

Will it help to clear out existing lead ? ie. use a slightly higher dose (1 oz./gallon) for a few tanks and then go to a maintenance dose (1/2 oz./gallon)

Thanks again,

Darren

[tcrv7]

Avblend and MMO have seemed to help my C 85 that stuck the #3 valve 18 hrs after I got the plane. We rope tricked it then 20 hrs later same valve hung. Started Avblend and MMO [Marvel Mystery Oil], yes I know its mineral spirits and wintergreen, but 249 hrs later the rope has'nt been back in that cyl or any other YMMV.