One day after adjusting the carb for the last time, I fired the bike up and it still wouldn’t run anywhere near smooth. It ran like it was misfiring every other compression stroke. I let it idle for a few minutes and suddenly there was a loud pop and then it started running perfectly. I ran another minute or so and then white smoke started coming out of the exhaust.

I immediately killed it and checked the oil. When the dipstick was removed, nothing but milky oil came out. All of the coolant, and I mean all of it, was in the oil. I proceeded to remove the head because that is were the pop sounded like it came from. Once I got the camshaft out there was a big hole underneath one side of the cam where the lobes are. One of the two hex head plugs blew through the head toward the cylinder.

I have searched for hours for anything related to this issue and I have come up with nothing. I also happened to notice that there is a Raptor 660 head for sale on ebay with the same spot where mine has a hole covered with JB weld so I feel like mine could not be the only one that this has happened to.

I am very experienced with small engine work from outboards to four wheelers (especially Honda four wheeler which are very different) and I am lost on this one. Does anyone have any idea on what could have caused this?