Some years ago I took my B12 with standard Ivan mods, out to New Mexico, from New York.
As I went through the center of the country and into Kansas, the bike started running a little notchy, sort of missing at low throttle opening. I checked the air filter... LOOKED clean enough.... then subsequently took the carbs off at a motel near Colorado... they looked pretty clean.
Took them off AGAIN into the mountains, this time to move the needles a notch.. maybe the thin air ??
Nope. Into Santa Fe in the middle of a nightime, frightening thunder storm, and in the morning the bike would hardly start. Belched out some black smoke. I limped it over to the local Zook dealer, who had no idea and suggested I needed heavy motor work. I gritted my teeth and took the bike north.
Finally, around northern Colorado, I swapped the air filter for a new one, even though the old one just didn't appear DIRTY, just a little GRAY.
BAM! The bike suddenly ran like a raped-ape...... and the rest of the trrip was a smooth gas.
Change your air filter!!