Engine Misfire at Altitude.

[basloane]

We have a 1974 T337G. In late 2011 we installed a Factory Remanufactured engine in the front position, and reinstalled the ( incumbent ) magneto pressurization kit.

First flight up to FL210, we experienced misfire "kicks" above about 17,000 feet.

We checked out both pressurization systems, confirmed that they allow air to flow and that they will hold pressure. On the next flight, we experienced the same issue above 16,000 feet.

Then,... one day I noticed that the pressurization systems were drawing air from a fitting located on the venturi throat of the cabin pressurization educator. We tested it with a pressure gauge and ( sure enough ) it would draw a vacuum when we ground ran the engine.

We relocated the pressure tap to an area where we were sure that we had high pressure air, and believed we had solved the problem. On the next flight however while climbing up to FL210, same problem while climbing through 16,000 feet.

We then attempted to identify a specific magneto. On the front engine, we experienced very rough operation at altitude on the right magneto. Other magnetos seemed fine. We dismantled the right magneto, found a bit of oil and pitting on the rotor electrodes but otherwise nothing significant. Cleaned it up and put it back together. On the next flight, while climbing through the 15,000 to 16,000 range, same problem ( kicks ) on the same ( front-right ) magneto.

At the annual inspections, all plugs are cleaned, re-gapped, tested and re-installed.

Same problem continues starting at 16,000 feet.

Has anybody experienced this problem? This one is driving us nuts...