Looking for some help determining what exactly caused my detonation and ultimately pre-ignition. Motor is a bone stock 650 triple. Was 15-20 deg day on a lake, raced my buddies xc700 for 1/4 mile, then cruised at 50 for a 1/2 mile and pinched her for another 1/4, rolled off burped the throttle a few times, and pinched her again and thats when she melted.

I'm in the process of rebuilding the entire motor. Waiting to get my crank/cases back from Northern crank. Something tells me the crank was twisted causing my issue based on how the mag was perfect. Mag wont "twist" in regards to ignition timing. Then again the mag does run richest due to the jog in the airbox for the steering shaft so maybe thats what saved it.

I'd like some opinions based on what the pistons and plugs look like as to the true culprit. To me it seems it wasnt running too lean based on the mag piston wash, the exhaust side skirts, and plug color. I realize the skirst are scored, but I read somewhere, when its the edges of the exhaust ports thats its a result of the piston swelling from pre-igntion. Again this is a stock 650 with stock jetting.

This motor has a history of burning center pistons. My dad told me, same thing happened to him on a lake years ago. This motor has been in the family since 96. It was my first sled. I remember my dad blowing it up but I dont remember the details as I was young. Motor was decommissioned in 08 and resurrected last year in a 95 440.

Id also like to know, has anyone ever been able to audibly detect detonation? Im very familiar with 4 stroke ping but I have never heard a 2 stroke ping.