Some popping on decel is actually a sign the bike is set up right so you might go back an 1/8th turn at a time until you're happy. Your elevation really isn't high enough to play into it as I routinely go from 900 to 2000 ft on a daily drive without drama. Now, when you say the plugs are black is this the insulator (where you're meant to be checking) or the base of the plug (which will blacken at tickover and not clean itself for many miles)?
