Black plug
- Thread starter supercub1999
- Start date
supercub1999
Registered User
It's an O-320 and Phillips XC 20/50, I'm wondering if the rings might be bad on that cylinder, a year ago on the annual it had 66 compression and the other 3 were mid 70, the plug wasn't black like that though last year, looks like it's getting worse, engine has 1300 hours on it over the past 20 years and I put about 40 hours on it since the annual
Steve Pierce
BENEFACTOR
Graham, TX
Doesn't look wet like oil to me. Looks like it is running rich.
skywagon8a
MEMBER
SE Mass MA6
It looks like the beginnings of oil consumption. Where did the air leak out during the compression check? Carburetor (intake valve)? Exhaust (exhaust valve)? Breather (piston rings)?
Last edited:
skywagon8a
MEMBER
SE Mass MA6
In one cylinder?
CamTom12
Registered User
Huntsville, AL
I was thinking the same as Steve and then thought about this. Maybe with induction leaks in the other three cylinders, leaning them out? Probably not likely, though.
I've always seen oil in the plug as wet, but maybe what I've seen has been more developed than what he's got going on. It'll be interesting to see what the cause ends up being, for my own mental toolbox.
Good luck, OP.
The time I had heavily carboned plugs in one cylinder with low compression, it was a bad plug. Compression was going right through the plug and the center electrode was flopping in and out. But you would have caught that problem because there was exhaust carbon on the spark plug lead. jrh
wireweinie
FOUNDER
Palmer, AK
supercub1999
Registered User
This is the one cylinder that doesnt have a primer line, the other 3 do. Going to try and listen for the source of the leak right now.
Thanks
I need to keep posted on your results. We installed an engine analyzer in our PA-12 with a 160hp O-320. The front two cylinders run at about 1200 EGT / 250 CHT while the back two cylinders are running 1450 EGT / 380 CHT. We too thought that the primer lines might be the issue since they are only hooked up to the front two cylinders. But, after capping the lines, there was no change during the test flight.
skywagon8a
MEMBER
SE Mass MA6
Frodo,
Your temps indicate that the rear cylinders are leaner than the front two. This is likely due to the normal carburetor installation. When the butterfly valve is only partly open the airflow is disturbed enough so the fuel/air mixture gets uneven flow distribution downstream of the valve. The mixture distribution is best only at wide open throttle. A fuel injected engine will differ since the fuel mixes with the air at the intake port of each cylinder thus minimizing the differences.
Your temps indicate that the rear cylinders are leaner than the front two. This is likely due to the normal carburetor installation. When the butterfly valve is only partly open the airflow is disturbed enough so the fuel/air mixture gets uneven flow distribution downstream of the valve. The mixture distribution is best only at wide open throttle. A fuel injected engine will differ since the fuel mixes with the air at the intake port of each cylinder thus minimizing the differences.
algonquin
MEMBER
Seldovia,Ak
I think I would have investigated the low compression during the annual. Lycoming recommends anytime there is a diference like that it should be looked at to include pulling the cylinder if you don't find anything. I would guess a exhaust valve burning. If you were getting popping a stuck valve. If it's rings check you oil filter / screen for chunks of metal. Do another compression check. Good luck with it.
supercub1999
Registered User
Thanks for all the advice, havent been able to have the mechanic do a compression test again yet so I rotated plugs to a different cylinder and took it flying, runup was good as it normally is, and adding power on takeoff it pulled hard like normal and sounded fine. It's not using oil, a pint every 5 hours at most. The mechanic said to keep running it. He had that cylinder at 68/80 on the annual last year and the other three mid 70s /80. Will be doing another compression check soon.
skywagon8a
MEMBER
SE Mass MA6
I agree with your mechanic. Let us know if the "new" plugs in that cylinder start to color up. I am thinking that with 1300 hours that those rings are just starting to let a little oil by. Not enough for the plugs to be wet nor enough to reflect in oil usage. It wouldn't hurt to look through the plug holes with a small mirror/light or a borescope for scratches on the walls. That would indicate a broken ring which would not necessarily reflect in the compression numbers.
