Sorry for being tardy replying. I didn't realize that e-mail notifications are off by default.
Robski wrote:do you have a misfire or not ?
The pico seems to be telling me that the misfire is a 'system' issue and not isolated to a single cylinder. We see too-high firing lines randomly across all cylinders. The misfire is evident by ear and by eye. The evidence of misfire is not as evident during warm-up. Thus, a rich mixture somewhat abates the symptoms. But, even a super-rich mixture does not cure the primary issue.
Diagnosing a misfire is a second-order diagnostic. We are actually attempting to diagnose a harsh harmonic vibration that occurs at 1250 RPM increments with severity increasing with load. So, we are looking for mechanical issues as well.
I could write several pages on the steps that have already been taken with respect to both mechanical and engine-operation diagnosis. Acquisition and use of the Pico system is the first step in looking for the more-bizarre failure modes. More-correctly, the Pico system is an upgrade from an old, poorly-operating 'scope that wasn't up to the task.
The car in question is - as might be evident from my user name - a Porsche 928. This 928 - an '84S (ROW not US/NA-spec) - is old but has been extremely well-cared for. It has rarely seen rain and has only 43k-miles on the odometer. Virtually every component attached to - or part of - the engine that might be subject to degradation due to time or mileage is new or less than three years old.
We'll be attacking the problem again tomorrow looking for evidence that might point in the right direction.