I PUT A JET KIT IN MY 06 YFZ450 AND NOW IT RUNS LIKE CRAP IF I HOLD THE THROTTLE HALF OPEN IT WILL START BUT IT BACK FIRES CAN ANYONE TELL ME WHAT IS WRONG AND HOW TO FIX IT......  

i no some people with that problem and they rejetted it and put a rev box/cdi box but if u have a full pipe and needs jetted, jet it to it's recamended jet sizes  

have you modified the exhaust at all?
upgraded or changed it or removed the baffles?

if not, rejetting wont make much difference and you'll run into this problem.
maybe remove the baffle so the exhaust can breathe better. you'll get more hp out of it that way as well.

you are either not getting enough fuel or getting too much.

you could try turning the fuel/air mixture screw on the bottom the one that has numbers on it.

if you rejetted larger then you'll need to compensate for the extra fuel by turning the air/fuel mixture screw out a little to allow more air into the mixture.

if you rejetted smaller ( no one ever does this)
then turn it in a little at a time this will increase
the amount of fuel in the fuel/air mixture.

as well anytime you rejet, the carb has to be retuned.

but yank the baffles out if you haven't upgraded to a better exhaust system.  

welll did u just put a jet kit in it for no reason?? or did u ad a pipe?b/c if u just added a kit with no mods done to the pipe or airbox....its probaly getting to much fuel or not enough try or tey the fuel air mixture screw  

YFZRACER216:
I PUT A JET KIT IN MY 06 YFZ450 AND NOW IT RUNS LIKE CRAP IF I HOLD THE THROTTLE HALF OPEN IT WILL START BUT IT BACK FIRES CAN ANYONE TELL ME WHAT IS WRONG AND HOW TO FIX IT......

Help for YFZ 450 jetting............

If you pipe the bike the hot setup in SOCAL at low altitude is GYTR 48 pilot, 168 main jet, with 5th from the top on the needle clip. This is rich for sand dunes at low elevation. If you buy the GYTR kit it has instructions which you should follow. It will probably suggest 1 setting leaner all the way such as a 165 main and 4th from the top for the clip. I bought my 2006 used with a GYTR pipe and dynomax jet kit (they are junk)we changed it to the GYTR jet kit (which is cheaper). It ran poor when cold. Now it starts immediately and runs full power cold, hot all the time perfect and flawless. My bike also has the K&N air filter and K&N air box top with velcro nylon over the pleated air box top. It is a great setup.

Good luck, tuning is everything.
Reed 619-954-8416