My 450F stuttered (acted rich) at 1/4 constant throttle, 1st and 2nd gears. The dealership could not correct it and claimed that it was natural for the bike. They even offered me a ride on one of the floor bikes. It sounds like jetting huh. But, I had tried all the stock needle clip positions. I even tried a leaner needle but could not get rid of it. With the Throttle Position Sensor (TPS) unplugged, the problem was greatly improved. I replaced the CDI with a Vortex CDI ignition that has 10 maps. The problem is gone in half the map selections. Horay! Vortex said that the stock CDI has troble interpolating between the curves and causes the spark plug voltage to drop. This made sense to me because on my WR250 acted rich when the battery drained down and the lights were on. Thus, it's CDI could not keep a hot spark. I guess the mx riders never use constant throttle and as I get better on the track I seem to notice it less. However, out in a field or trail style riding that stutter was a pain. Plus, since I just got it, I was increasing speed gradually. I was afraid to really crack it. All the Vortex traction maps were fine. One performance map did not have the 1/4 stutter. Here is the puzziling thing. What is going on that timing can screw it up like that? Why not just keep a constant spark? Why doesn't Yahama fix it? Could there be another issue going on? It is surprising that timing can make you think you have a jetting problem! I am very happy with the Vortex CDI since it fixed the problem and gives me several different curves.