WEIGHT... and the fact that MX bikes have their necks wrung most of the time= RACEBIKE A racebikes engine lives in a much narrower RPM range than a streetbike, so jetting for premix with a engine that mostly lives between 10 and 14k is optimum. On a streetbike who's engine live's between 2k and 12K depending.. With Premix, you will either be over-oiling when ridden mellow, or potentically even under-oiling if you are not running a rich enough oil mix (say 80 or 100 to 1). Most oil injection systems are "Variable Rate", delivering more oil as your throttle is opened and RPM's increase. And a 2 stroke street bike is ANYTHING BUT a lazy man's bike. You are always putzing with something.... And funny, of the 50 or so 2 stroke street bikes Ive owned, I cant remember putzing with oil injection pumps other than initial set-up after building an engine? Maybe I (and millions of others) have just been lucky.