[77newell]

The ground wire should be sufficient provided it's working properly and on old coaches that is sometimes questionable.

Given your improved results it seems I was probably mistaken about the ground. It seems more likely that excess carbon from the disturbed flame was gathering on the first cooler object it hit and the more it collected the more attractive it was to further collection. There may be more carbon collecting further up in the system so when you have the time and inclination it might be worth checking, though given the improved performance you are getting it's probably not a big urgent problem.

Thanks for sharing what you did and what resulted, that helps us all in the future.