Spark Plug question (but hear me out)

[Doug]

How did you eliminate the 'magnetos as an issue'?
If this was simply by way of a test then it is unlikely to have given you an accurate result especially if the mags are marginal. Tests are not absolute - false positives and false negatives are a reality.
If you replaced the mags in their entirety with known good units and found the same symptoms with the 'new' plugs. - then I agree the problem is very odd.
If you did less than replacing the capacitor and coil then I don't think you can eliminate the mags as an issue just yet. Possibly the lower resistance of the 'new' plugs is loading down the power source (the L & C) within the magneto shortening the spark duration to a point where the cylinder does not fire - especially at low RPM (ie less energy going into the L&C) during start.

[F1R]

Try a set of UREM37BY I have always got better starting with the exposed electrode plugs versus the recessed electrode plugs. They are much more forgiving of less than perfect air fuel mixtures also.