There were a couple of things about the engine that troubled me on the way back from Yarmouth on Tuesday. The rate of climb was noticeably lower than expected above 6,000 ft. And the #2 cylinder Exhaust Gas Temperature (EGT) didn’t vary quite the same as normal when I leaned the mixture in cruise. I hoped that I simply had a partially plugged fuel injection nozzle, which would cause the #2 cylinder to be running too lean, and the others to be too rich during the climb (I lean during climb to keep the highest EGT at the value seen during take-off, as recommended by Advanced Pilot Seminars and John Deakin).

This morning I pulled the cowling and inspected the fuel injection nozzles. I couldn’t see any evidence of a partially plugged nozzle, but I cleaned them with an ultrasonic cleaner, and reinstalled them in the same cylinders they came from. I checked the air filter - it was in good shape, but I cleaned and reoiled it. Still looking for the smoking gun, I did a differential compression check. Three cylinders were good, but #3 was worryingly low at 63/80 (normal is in the 70s). Air was felt to be coming out the exhaust pipe, which means the exhaust valve was leaking. This cylinder was 74/80 on the previous compression check, but it was the only cylinder where the escaping air was coming from the exhaust on that check.

I removed the rocker cover and “staked” the exhaust valve with a dowel and a hammer to open it and possibly dislodge any foreign material between the valve and valve seat. I checked the compression again, and this time it was 60/80. The compression check was done cold, so I would need to redo it after flying the aircraft to warm up the cylinders to confirm the result. But, I had found a broken flange on a heat muff inlet SCAT hose, which I would need to fix before flying.

I still hadn’t found the cause of the too low climb rate, as a leaking exhaust valve wouldn’t have much effect on power, so I decided to cut open the oil filter from my last oil change. I had meant to inspect the filter a few days after the oil change, after it had drained, but that slipped through the cracks. After cutting the filter open, I was very dismayed to see quite a bit of ferrous metal in the oil filter. Every pleat in the filter had several dozen very small pieces of metal. The filter from the previous change had been almost completely clean, so something had happened in the 50 hours between oil changes.

Lycoming has published a long list of inspection criteria, based on the quantity, size and type of metal. I’ll acquire some solvent tomorrow to rinse the filter media so I can quantify the amount of metal. I’ll also take a look in the filter that has been on the aircraft for the 20 hours since the last oil change before calling Aero Sport Power to get their advice. I think the next step is to pull a cylinder or two so I can inspect the camshaft lobes. Given the amount of metal in the filter, I fear the engine should be disassembled for an inspection to find the cause. I would also need to flush the oil cooler and lines, and possibly have the prop governor overhauled. The prop would also need to at least be flushed - I’ll need to talk to an MT prop shop to get advice.

I’ve decided not to fly the aircraft to Oshkosh this year for the big fly-in at the end of July. I’m going to be on the road for a week between now and Oshkosh, and that doesn’t leave enough time to get things sorted out. There are too many long stretches with nothing but trees between here and Oshkosh to risk flying there before I get to the bottom of things. It’s going to be painful doing that drive.