Just did a complete new top end on my brother's 01 CR250. It ran good for about 2 tanks of gas, and now it has a severe miss in the low to mid range, kind of like it's loading up real bad. On the top end it runs excellent, just like it's supposed to. So far we looked at the exhaust valve, and it seems to be opening and closing as advertised, pulled and cleaned the carb, and inspected the reeds. Also put in a brand new plug. Everything seems okay, and we're pretty confused. Any help is greatly appreciated. Thanks!