Well, on the Keihin PJ, the choke circuit is like an adjustable pilot jet. So, if it's turned in too far, the bike will be lean as you roll on the throttle and bog, especially after a long period off the throttle (like on a dowhhill). If the choke is screwed out too far, the engine will fire periodically as you decelerate and cause it to buck or jerk. These are my experiences with my properly jetted Keihin PJ. Since his profile says he has a 1985 CR 125, he probably has a carb similar to the Keihin PJ on my 87 CR 250, except with a round slide instead of the flat.
Don't believe me, this is what Honda says:
Remember, this is a 1987 CR 250, I don't have the manual for the 125, but I'm sure it's close.