Not sure what kind'a carbs yer runnin' but if they're CV carbs, check the integrity of the slide diaphragms, looking for cracks in the rubber or leaks around the outer seal. If the diaphragm(s) is(are) leaking, you lose vacuum and the slide won't open normally as you crack the throttle, leaning your fuel/air mixture. If I was a guessing man, I'd look at the two carbs that weren't running rich first. Prolly the two cylinders that looked rich were carrying the load on acceleration. Some other things to check: if your carbs have squirters (most bike carbs don't) make sure they're working; re-check the orifices on your main jets for blockage. Edit: Also, check yer other vacuum hoses, crackcase breather hose, etc for cracks/leaks.