There was another post about this. It has to do with the ability of a high voltage spark to jump through air. At high altitude there's less air and there needs to be more of a voltage to jump the same size gap.
I'm sure there is an electrical nerd somewhere cringing at that explanation but it's the best I have.
Yep. I am. You have the spark jump theory exactly backwards.
Air insulates against spark jump. So at altitude spark will jump a given gap at a lower voltage.
Other choices are: incorrect fuel/air mixture, insufficient ions created in the spark to propagate flame, spark jumping early and not building up a nice fat arc (this is my most likely pick). Might be some other reasons I can't think of at the moment.
Unimogbert
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