Hey Perry, Ted (the prop guy) here.
Again...not my motor, but we've tried all the logical things.
Swapped ignitions from another BME that didn't have a problem.
That didn't fix it. Switched gas/oil to Mobil 1 MX2T (32:1) that works fine in my ZDZ and 3W-100. BME ran a little better but still sagged 6 minutes into flight. Took the cowl completely off. Engine ran much cooler (200F instead of 260F), but still sagged before 10 minutes of flying. Set the high needle 1/4 turn richer on the ground (also tried 1/8 turn richer), and the engine just ran without power in the air (no vertical).
I will bet a dollar to a donut that it is a mechanical problem or a fuel delivery problem. The engine does not turn over as freely by hand as other twins (tight bearing?, twisted crank?) and the nose of the crank is wet with oil (likely bad crank seal) - not a lot of oil, but I don't like to see any oil.
This is not a BME bash. The owner has a BME-76 that runs great.
Len, good thoughts on the fuel tank. If the tank isn't vented properly it could just get leaner and leaner through a flight. We'll check the fuel system out.