My yfz starts right up and idles just fine, but once you start going at probably anywhere from 1/4 to 1/2 throttle it wants to bog down. When you stuff your thumb into it it stutters but then gets going fine. Is this a carb problem or what? thanks for any replies

 

Very, very, very, very common problem with the YFZ450s. Lots of people have little things you can do to help solve it. In my case, I just learned to ease in the throttle at first, and then punch it. I think there is a different "leak jet" you can also purchase to help solve the problem, but I've never done it.