I have a 2004 YFZ450 . Mods are FMF full system with PowerBomb header . UM aluminum airbox ( I run with the lid off ) that use's a TRX250R size K&N airfilter . I have taken it to two certified yamaha techs and told them about the problem and still it runs like crap . 1st and 2nd gear runs ok . 3rd starts to pull like crazy till at full throttle 4th and 5th from half to full throttle it sputters and feels like its wanting to cut out on me . any help would be great .

Thanks

 

yea ur runnin too lean
with those mods you definately wanna rejet
you are goin to have to get a bigger main jet

 

You may want to also move the clips on your needle to allow more fuel when you get to full throttle.
It does sound like your leaning out.

Don't run it too long before making the changes.