i am noticing that when i start my 1999 YZ400F it behaves fine and is easy to start has plenty of compression, has plenty of power, ect. but if i rev the motor before its warm it smokes a little, not a huge plume or anything just a little, if i dont rev the motor untill it warms up there is no noticable smoke. have any of you had this before? is it normal or not? give me the run down. thanks in advance.