This is just coming from automotive experience so take it for what its worth, but the problem is the same volume of air running through the carb having a lower concentration of oxygen due to the higher altitude. You need to lean out your fuel mixture. I don't know much about mules so I can't really tell you how to do it on your particular carb but thats the gist. I'm sure someone here (or the manual) will give you the info you need.
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