Suddenly low compression on one cylinder - time to pull it?

[rocketbob]

I put in whole quart bottles of MMO. A pint isn't enough to do much.

[Glas467]

Did you check the suction screen during the recent oil changes ... Agree with the above that broken rings would likely result in noticeably increased oil consumption and dirty plugs ... But the screen would have caught any larger pieces of broken ring or other metal. Also, how have the filter inspections looked, any more than your previously normal amount of debris in the paper pleats?

Reggie

[Mike H]

Originally Posted by rocketbob

I put in whole quart bottles of MMO. A pint isn't enough to do much.

Why not just dump in a half quart of mineral spirits and save yourself some money!
http://en.m.wikipedia.org/wiki/Marvel_Mystery_Oil

[SHIPCHIEF]

68/80 is a noticeable leak. Put the compression tester back on that cylinder, remove the oil dipstick and listen at the dipstick (oil fill) tube, then the carb air filter, then at the exhaust pipe.
It's coming from one or all of those, and no where else.
If you have an intake valve leak, well that's rare, and the end of the line, the cylinder should probably come off. Loss of an intake valve, or even being stuck open is instant loss of power for a carburated engine, and nearly as bad with fuel injection.
Piston rings rarely align their end gaps, but would cause a big leak. Rings eventually lose their seal and hot gas blows by, causing an increase in oil temperature, and maybe you would notice that one cylinder head temp has been increasing? (if you have individual cylinder temp gauges) This might be visible thru the spark plug hole with the piston at Bottom Dead Center. The cylinder wall would be brown from exhaust / heat staining. Or you might see some scoring. Sometimes a bit of sand or broken piece or ring, or a micro weld can start scoring. Scoring never stops, so you take the cylinder off and fix it if caught early.
Has the exhaust temp on that cylinder been changing? That and a hiss at the exhaust pipe.. well you get the idea.

[walkman]

Originally Posted by PerfTech

... Before pulling the cylinder, you may want to check for a tight valve as this will act much as you described and generally get worse with heat at idle. Thanks, Allan...

Just re-read this.

By "tight valve" do you mean one where the clearances in the valve train are too small, that is, pushrod effectively too long?

There's enough clearance cold for the rocker to move when the valve is closed, but I have not done a proper test of that. Is it possible that the valve stem could have stretched or something?

[walkman]

Originally Posted by Glas467

Did you check the suction screen during the recent oil changes ... Agree with the above that broken rings would likely result in noticeably increased oil consumption and dirty plugs ... But the screen would have caught any larger pieces of broken ring or other metal. Also, how have the filter inspections looked, any more than your previously normal amount of debris in the paper pleats?

Reggie

Have not checked the screen, will do when I change the oil Cut open filters are clean. Nothing but a tiny amount of carbon, nothing sticks to a magnet.

[rocketbob]

The valves stretch in length and the stem gets smaller in diameter. This is normal and expected. The clearances are way over the amount that this would affect the valvetrain unless the engine was assembled at the tight end of the dry tappet clearance.

[walkman]

Originally Posted by SHIPCHIEF

68/80 is a noticeable leak. Put the compression tester back on that cylinder, remove the oil dipstick and listen at the dipstick (oil fill) tube, then the carb air filter, then at the exhaust pipe.
It's coming from one or all of those, and no where else.
If you have an intake valve leak, well that's rare, and the end of the line, the cylinder should probably come off. Loss of an intake valve, or even being stuck open is instant loss of power for a carburated engine, and nearly as bad with fuel injection.
Piston rings rarely align their end gaps, but would cause a big leak. Rings eventually lose their seal and hot gas blows by, causing an increase in oil temperature, and maybe you would notice that one cylinder head temp has been increasing? (if you have individual cylinder temp gauges) This might be visible thru the spark plug hole with the piston at Bottom Dead Center. The cylinder wall would be brown from exhaust / heat staining. Or you might see some scoring. Sometimes a bit of sand or broken piece or ring, or a micro weld can start scoring. Scoring never stops, so you take the cylinder off and fix it if caught early.
Has the exhaust temp on that cylinder been changing? That and a hiss at the exhaust pipe.. well you get the idea.

I really appreciate all the insight and input from everyone on this thread and via pm. While I've built a few 911 engines, and got 25 years of Lycoming ownership and hands on, I still consider myself a backyard mechanic.

wrt to the comment above, listening to the engine is a bit of a black art that requires experience and a tuned ear. Back in the day I could put a screwdriver up against a 911 block, the other end in my ear, and tell you if it was rich or lean. No longer I'm afraid :-(

At this point, it doesn't make any difference if its a leaking intake valve, leaking exhaust valve, or broken rings. They all mean the cylinder's gotta come off. :-(

Cylinder walls are really greyish, perhaps with a tiny tinge of brown. Some appearance of vertical scuffing or scoring up near the choke, I don't know how much is normal. I have individual CHT and EGT, but it doesn't record. Nothing appears abnormal with those temps, but I haven't been writing them down and comparing over time.

I think my plan of attack is to do a quart or so of MMO again and dee what happens thursday night. Perhaps I'll find a local source for the lard and make my own :-)

If that doesn't help, I'm pulling the jug on Saturday.

If the rings are indeed coked up and not broken, whats the best way to clean them? I'm assuming oven cleaner is not the best direction.

[rocketbob]

I have put removed carboned pistons in MMO (think marinade) and the carbon will definitely loosen up. Wouldn't hurt to try putting the cylinder on TDC then fill it with MMO and let it sit for a day or two, then remove the bottom plug and drain it. Worth a try perhaps...I've never done this so YMMV.

[Mike H]

Originally Posted by rocketbob

I have put removed carboned pistons in MMO (think marinade) and the carbon will definitely loosen up. Wouldn't hurt to try putting the cylinder on TDC then fill it with MMO and let it sit for a day or two, then remove the bottom plug and drain it. Worth a try perhaps...I've never done this so YMMV.

If you do this use old plugs. If you saturate the plugs with penetrating oil or fuel for long enough they will develop an internal short and they may spark outside of a cylinder when tested, but they will not fire across the gap under compression.