Will,
have you checked the resistance of each of the leads, for comparison?
My guess is that the classification refers to the amount of RF suppression provided by a lead and that the Class C have greater overall resistance.
The fact that they didn't work too well with your coil also suggests that your coil's output may be borderline.
I had something similar with a Citroen 'special' recently, which had the coil mounted in a location that required leads to be made up.
Copper cored wire was used, with suppression provided in the spark plug caps. It wouldn't even start if caps with 10 kohm resistance were used, but started and ran well once these were replaced with 5 kohm type...
Dunno about leads for Amis, but the main difference between those suitable for a 2CV and those for a Dyane is to do with their lengths, since on a Dyane the coil is offset to one side instead of sited centrally.
ken.
( I'd never bin leads before testing both their resistance and insulation, the former with a multimeter and the latter with the 'water spray' test.
)
