Trying to decide on a new '07 mxer (riding an '05 WR450 now). I'm wondering if the YZ needs to be rejetted for the eaerly Spring New Engalnd weather (races start in March when its in low 30's). Is it fine to leave it alone? (my WR hesitates and coughs off idle). FYI I am a 40 year old vet novice. Any comments (besides that I'm an old fart)?