Initial thoughts would be "lean jetting".

Another thought would be "the rod broke" and slammed a hole through the top of the piston, but you would have had an immediate failure. You mentioned that it ran for awhile under low load, therefore it can't be the rod.

Again, my guess, LEAN JETTING.

 

are you running the correct plug? If you are using a longer plug that will do it too..but you knew that already.... right?

 

Well it wasn't lean ..It was acctually rich ..I had the air box lid of because I didn't change jets from winter to summer yet ..And it was jetted for Winter riding and it was perfect then ...Just before it died it blew out a crap load of oil from the crankcase vent tube...Drained the oil and nothing was stuck to the magnet when I "fished around" for broken pieces..But anyway thanks for the idea's It's just going to sit in the back of the garage until the 770 kit gets here.

 

Well it's apart now and looks very good inside the motor (even took the tranny apart to check that)besides the hole in the piston ..I figured alot of stuff out and found it only to be a faulty piston ..I had a full race season and a few trail rides on it so I guess it was about the life of that piston. the piston itself was a nice charcoal color ..not a deep charcoal but looked like a good running piston maybe a little rich..It loaded the crankcase with gas and oil and blew it out the breather..the coolant system works fine (aside from the temp sensor not working) and so does the oil system..the piece of piston must have went out the exhaust because I found nothing inside the motor or marks on the crank,valves or head. the sound I heard from the engine priar to it blowing up was the hole in the piston most likely loading up the crank with extra oil and gas (this would explain why I had a loss of power and fuel mileage on the trail rides a weeks before) so most likely the hole had been in there for some time before getting hot enough to get bigger..I noticed the oil smell almost two months before this happened (extra Oil & Gas being blown out the vent tube and dripping onto the motor) So it had been blown up so to speak for quite sometime! One of the reasons I never noticed any oil loss was because I changed it way to often due to tranny trouble and stuff like that. In a way I was hoping for a crank problem so I could get the new Powroll 770 kit ..but since the crank is fine I'll stick with the 686 I'm getting tired of taking apart good running engines and modding them only to have more problems with them!

 

It seems posible a single detonation may have made the hole, a plug or fuel probelem, good luck, with that it won't take much to have her ripping again.

 

Funny thing is ..It ran like that with the hole for almost 3 weeks I'm assuming since that is when it started making noise and burning oil.

 

It may have taken 3 weeks to burn the hole but I garuntee once that hole was there it stoped running!