That is not much to go on, does it bog or buck when you hit 20, but to me it sounds like it would be the cdi. Can you get it above 20 if you shift to a higher gear, and if so does it bog down again ? If this is the situation it would be a computer problem, if not i would check your spark maybe your not getting a spark to one of your cylinder.
Answers & Comments
That is not much to go on, does it bog or buck when you hit 20, but to me it sounds like it would be the cdi. Can you get it above 20 if you shift to a higher gear, and if so does it bog down again ? If this is the situation it would be a computer problem, if not i would check your spark maybe your not getting a spark to one of your cylinder.