Completed the rebuild of my 1999 KLX 300 about a week ago. Had the ACR retimed, added Bill Blue 331 kit, and had the head redone. Spent yesterday putting the complimentary break-in miles on it. Everything seems to be working as planned except possibly one thing. When you get up to around 40 mph a distinctive rattle resonates from the engine. I believe it is engine knock caused by the fact I now have 12:1 compression ratio and only have the ability to buy 91 octane fuel. Anyone else had this problem ? Any recommendations on a fuel additive in order for me to test this theory ?