I need HELP!!

your yz400 is also quite a bit older and quite a bit heavier then a CRF250R.

my previous bike was a CRF150R. that was a "straight up MX bike" and it took trails amazingly well. the suspension was a little harsh and the gearing was a little high but it worked very well given the fact that i could ride trails one day and then the next day hit the track.

Age doesnt have anything to do with it, as long as everything is in working order. Im sure that little 150 generates alot of heat.