yeah check your timing and point gap.
i was dealing with similar symptoms recently - 4 stokey low end and then fast when it got up to speed - i thought it had to do with gas/air ratio so i spent a ton of time trying different jets and needle settings with no change at all in performance. today i opened the point gap a little and it took right off.
even if you haven't touched that area recently, a different cylinder affects the length of your stroke right? which will in turn effects timing? maybe not, but either way different cylinders seem to have their preferences.