Well this kinda sucks. I just got back from the local dealership where I was told this sort of behavior is absolutely normal. You simply can't punch the throttle from an idle without the yfz hesitating severely. Mine usually just completely dies. Needless to say I'm going to try and re-jet it myself. I just got the service manual for it and I will be ordering a jet kit. If there are any yfz owners out there who have whipped this problem please let us know.
Thanks,
Tim

 

Mine does it too, but it wasn't due to being too lean, in fact I am currently too rich w/ the stock jets @ 4500ft elevation. I've played w/ the fuel screw and that helps a bit, but I don't notice it at all when I'm at normal running temperature.

 

Tuna , If you rev it up a little does it still do it? or is it just from an idle?
mine does it but i think it is from the excelerator pump pumping in to mutch gas at a low rpm. if the rpm's are up mine doesnt do it.
from 1\8 throttle and up it is fine.

 

It's mostly just from an idle, but it will also do it if your putting around in 1st or 2nd gear and you slam the throttle.

 

Try changing your pilot jet to a pj45,it helped me alot.

 

heres the deal, i don't care how you jet the yfz, if you stab the throttle hard right off from a idle, it will die, or sputter before it revs.
the reason being, there isn't as much manifold pressure on a four stroke like a 2 stroke. the yfzdoesn't have a constant velocity carb like a ltz400 either. you will get a sputter or it will die if you slam the throttle when idleing.
now that is not how you judge the jetting on a machine. the bike will not sputter or hesitate when under a load i assure you. and as long as it doesn't do it under a load don't worry about how it responds when idle.

 

That sucks. I'm considering getting the next year model YFZ (hopefully a full 450cc Yamaha guys) but this new carb issue might change my mind. I can live with the broken a-arms and subframes cause I don't wreck the hell out of my quads but I dislike very much having stupid problems like this one and electical problems on a new quad. Maybe I will go back to old faithful Honda and its great reliablity even if it might not be the most powerful or fastest.

 

does anyone know if the accelerator pump on the yfz quad be adjusted for timing and duration like on the dirtbike? i know that on the bike, especially the 426's, some improvements in off idle throttle response can be made by advancing the timing and shortening the duration of the accelerator pump.

it does sound like its lean though. you could try turning the fuel screw counter-clockwise in small increments to see if it helps. if the fuel screw ends up being more than 2.5 turns out, you should go to the next size larger pilot jet.

 

My 2 cents
I agree with KillerKX. Ride the bike and see how it responds under load.
If you still dont like it, take it back to your dealer. If they wont at least look at it, take it to another Yami dealer.
Theres no law that says you have to take your bike back to the same yami dealer you bought it from.
If you still dont get satisfaction, call Yami and let em know.
Im assuming its all still stock.

 

The only time it's an issue for me is when it's idling and I do it (intentionally).....other than that when I'm riding it's a non issue.