if you keep it all stock then run 87 or 89 octane. if you mod it run 91 to 93 octane. Adding higher octane makes it run richer and a stock quad is already rich so if you run high octane you will be running real rich and foul your plugs and loose HP. Check your manual in the trouble shooting sections. It will probably state that.

 

the higher the compression the higher the octane u need. my ds is 9.7:1 compression right now, and even on the gas cap it says to run 91 or higher octane. 450r's and yfz's compression is 10:1+ so i would definately run 91-93