My CR250 has an odd stumbling problem. After I warm the thing up, it stumbles in 1st and 2nd gear, but only the first time I use those gears after I start it. Ehen I grab 3rd, the bike runs perfect, and when I downshift back to 2nd or 1st, it still revs and delivers the poer great. It did it awhile back when it was stock then I decided to rebuild my entire motor, new reeds, carb cleaning and tune, and new exhaust system. Can somebody tell me what the hell is going on. Id doesn't affect my riding or the bike's performance, but I'm just curious. My 2000 CR500 doesn't do it.