low compression, bore scope pics, carbon...

[BruceEicher]

Routine oil change and FWF inspection, expected no issues...
But the compression test; 74-78/80 on others, cly 2 50/80 first few tests, rotated prop several times, got up to 64/80.
Late night did not listen for source of leak, but did take the time to scope with new Vividia VA-400.
Other info;
490 hour since new IO-360M1B
Always 100LL
Exxon Elite 20W-50
No fuel or oil additives
Engine has not missed a beat.
Runs smooth even in LOP cruise operations
EGT and CHT normal ranges and matched close.

Plan of acction:
Listen for leak, expect exhaust.
Remove rocker cover to perform rocker arm/valve stem tap to possibly clear some carbon.
Add Mystery Oil???
Test run, lean.
Run compression test again.
More aggressive leaning during taxi operations.

Any further advice, additives, oil choice or observations?
Thank you.

Cly 2






Cly 1



Cly3



Cly4

[MarkW]

Bruce,
I did not read the article but yesterday I received an email copy of the newsletter from Mike Busch (Savvy Analysis). He was talking about the ability and procedure to lap a valve without removing a cyl.
Right or wrong it is something to think about if this turns out to be valve related and there is no heat damage to the valve.

[pa38112]

The valves look good. The seats are a little dirty. What about the cylinder wall?
Find out where it is coming out first, then think about a course of action. There are three options: Intake, Exhaust, or oil sump. No matter which, I would think the first course of action would be fly it 10 more hours and re-test.

[bullojm1]

Originally Posted by pa38112

The valves look good. The seats are a little dirty. What about the cylinder wall?
Find out where it is coming out first, then think about a course of action. There are three options: Intake, Exhaust, or oil sump. No matter which, I would think the first course of action would be fly it 10 more hours and re-test.

++++1! The same thing happened to me last year. My #1 cylinder was reporting 64/80 when all others were in the mid-to-high 70's. My A&P friend told me to run it for 10 hours and retest. When I did, it was back in the mid-70's (and continues to be there...I am in the middle of my annual now the cylinders compression is still in the mid-70's).

[BruceEicher]

Originally Posted by pa38112

The valves look good. The seats are a little dirty. What about the cylinder wall?
Find out where it is coming out first, then think about a course of action. There are three options: Intake, Exhaust, or oil sump. No matter which, I would think the first course of action would be fly it 10 more hours and re-test.

Cylinder walls look bright, no scoring, cross hatch marks clearly visible.
Thanks for asking.

[Geico266]

Had the same issue on my IO-540. #4 was 30/80 at TDC. When I moved the piston back 1" it was 70/80. I flew it for an hour, no change. Flew it for 2 more hours, no change. Pulled the cylinder and had it re-honed, new rings, re-worked valves, 78/80. The top of the cylinder and side of the piston was coked up. Runs like a champ now.

[TCONROY]

I had low compression on a cyl last year and spoke with an A&P who gave me the same advice. Said the rings could be lined up, causing a leak. Retested at 5 and 10hrs and compression was back to normal.

[EXflyer]

Originally Posted by TCONROY

I had low compression on a cyl last year and spoke with an A&P who gave me the same advice. Said the rings could be lined up, causing a leak. Retested at 5 and 10hrs and compression was back to normal.

Never have seen them line up by themselves only when someone doesn't do there job. Also if the did you would be blowing oil out of the vent and or using bunches in that one cylinder along with plugs probably fouling.

[scsmith]

Your valve looks just like mine, no issues, EXCEPT, what is going on at the very top of the first picture? Maybe that's just a shadow? It doesn't have the halo colors of a burned valve.

fingers crossed this clears up by itself, and there but for the grace.....

[rvbuilder2002]

Originally Posted by EXflyer

Never have seen them line up by themselves only when someone doesn't do there job. Also if the did you would be blowing oil out of the vent and or using bunches in that one cylinder along with plugs probably fouling.

Do a googles search on "piston ring rotation"

It is pretty well accepted (and proven with some special testing) that pistons rings can and do rotate during engine operation.
That doesn't mean that all do, so staggering them during installation may still be a good idea, but it does mean that just because they are found aligned, that the installer didn't necessarily cause it.

Bruce, my recommendation is to not even bother with the wack it procedure.... just fly it for 10 hours and recheck. Most likely it will be back to normal.
I also noticed that spot on the edge of the first valve in the first photo that I think Steve is commenting about. Is the second photo looking at the seat underneath that spot? If so, it looks like there could be a large chunk of carbon there which might be causing the coloration difference on the edge of the valve at that point.