i just bought a brand new 07 yfz 450 got it home and started it up let warm for about 10 minutes then rode it has terrible flat spot in it down low if you gun the throttle from an idle it falls flat and sputters and backfires i have read all kinds of how to fix it blogs about jetting removing air box lid and baffle it is completely stock i know this machine is made to rev and top and but it is a 450 four stroke and should have some lower end grunt where should i start wolf1  

well if you are wanting to try to keep it stock, start with the fuel/air mixture screw.  the one with numbers on it, on the bottom.

just mark where it is set now with a marker.
turn it in 1/4 turn increments, and keep up with how many 1/4 turns you take it one way or the other so you can get it back to factory if needed.
with each 1/4 turn ride it for 5 minutes for the change to take the complete effect.
it takes time, but once you have it dialed it will fly!

does it have any problems on top end?
if it bogs down on top end then you are getting too much fuel,  if it cuts out at top end then you are getting too much air.

perhaps the easiest way to determine this is to take the airbox lid off and run it for about 5 minutes, see if it gets worse or better.

if it gets better then you'll need to add more air at the fuel/air mixture screw.

if it gets worse then you need to add more fuel at the air/fuel mixture screw.

yes these are awesome machines!!  designed with racing in mind and not having to spend a fortune to get it race ready. Yamaha really thought about the sport atv customer with this model.

if you aren't wanting to keep it stock then either remove the baffle or buy a good aftermarket exhaust like fmf makes, removing the baffle will allow it to breathe alittle better and give you a few more horses.
remove the airbox lid, and rejet it.
you can buy a rejet kit that has several different sizes and choices for jets, so you can really dial it in.
they sell for around $60.00 here.

Because Yamaha made this thing virtually race ready alot of people do experience issues running it stock.
but you should be able to dial it in close and keep it stock, but you may have to go up one size in jets.
But Yamaha really saved you a butt load of money in what they have done to the Yamaha.

it's really a very enjoyable sport atv once you get it dialed in.  

Hey wolf1,
Sounds like your the man to talk to. I just jetted my 06 with a 168 main, 45 pilot, GYTR needle #4 clip, GYTR slip on pipe, but, like others, a bad bog at the hit of the throttle. I was told 2.5 turns on the fuel mixture screw. We couldn't locate the mixture screw, right or left side? Do you suggest the aftermarket adjustment screws? Thanks for letting everyone pick your brain. It's nice to have someone who shares info so willingly. TG  

neither,

the fuel air mixture screw is located on the bottom of the carb

any screw with numbers on it. but you can get around that.
although most of the yzf450's come with numbered air/fuel mixture screw.

if it's bogging down then you are getting too much fuel.
if it cuts out then you are getting too much air.

however once you get it dialed in i think you'll be very happy at the throttle response and strength of this machine.

however it may not necessarily set at 2.5 with the jet kit and the exhaust. mark where it is set factory now
and work from there. just do 1/4 turn increments until you get it dialed. and ride for a few minutes between 1/4 turns to let is set in to it.

i never change the adjuster screws myself. i use factory and if i need something more then i usually go buy a larger d slide carb for a crisper throttle, the up side is so much better with a d slide, but you can get a good crisp throttle response with that factory carb.

the yzf450 was designed to be basically race ready.
a few adjustments and you have a seriously stout machine.

toss the airbox cover and remove the baffle or change the exhaust and you are ready.

and no problem i still love my job as a cycle technician and shop owner.

and the viewers here are absolutely awesome!!

Ray  

Thanks Ray, I do love the power of this thing. I'll play with it some more and get her dialed in, Glamis next weekend. Thanks again, TG  

if i had known 3 weeks back i could have stopped by and tuned it for ya.
i was in california for a good long vacation.