The plug itself was showing a pretty good mixture setting. You can mix the Motul at ratios of 70-1 to reduce the soft carbon build up on the plug. What I see here is consistent with a 50-1 Belray mix on plugs with 100 plus hours on them so overall it's good.

The reason for the "stumble" is because the engine is still cold. You stated that after it idles for a bit all is well, so that was a key statement. Moving the thermocouple to the back of the engine but in the same relative location on the fin will give a more accurate heat reading.

Pat