I have a 03 300ex I have a full pipe,jet kit and a k&n on it would it run OK with the air box lid off or will I have to rejet. ?

 

You should rejet for optimal performance, otherwise you will have a lean situation....which causes excessive heat build up among other things...this heat can melt a hole in your piston...alittle time rejetting really pays off...it'll run better and last longer...