So my '15 yz450f with less than 20 hours on it will start up but struggle to idle. It will run great as long as the idle is turned up and it runs hard for as long as you want it to but smokes a little, I suspect I sucked a little sand through the filter and &%$#@!ed my top end. The odd part is it WILL run and pretty decent until I cut it off even for a brief second then I star it and it runs like complete crap. Sounds like it is missing and popping. Here is a video link. It take a little while for it to act up (other than sounding like I need rings.) https://youtu.be/sjW801aWHLY I'm no expert mechanic but I am experienced enough to know there is more to the story than the top end. Took it to my dealer and they say nothing is reading wrong on the computer. They say the top end is causing it to run like that. But I don't understand why it would go from running hard and decent to complete crap after I shut it off and then start again. Please help because my local dealer could not. I've had a few opinions and they all say it is something electronic but no one can say for sure what it is.