I believe azvmax has the answer. I had the same problem with my yz400f. I went through the trouble of replacing the cable. When that didn't work, I accidentally figured out that I had the throttle grip assembly pushed to far down on the handlebar. The spring is not strong enough to snap it back when the rubber of the grip is dragging along the handlebar. I took it off, cleaned all of the smut out of it with kerosene, then re-attached it to the handlebars, making sure I let it hang over the end some, or at least enough to allow it to snap back. Check it the grip assembly for debris as well. Mine had gunk in it from previous applications of chain lube.