yes those holes are the relief holes so we can eliminate that. do Like Sam250 said inspect that powervalve so we can eliminate that as well. should no way hit but lets check anyways.if it hit would of destroyed right away and broke cylnder or something. then go to waterpump and check fins on impellar etc. I personally think either got hot from cooling or was to lean. I know you looked at plug and looked ok but you really did not do plug test you looked at it after which does not tell anything. With Ported cylnder,Rad Valve and better air fiter probabaly lean unless you did do plug test and jetted accordingly.

Not to cap on your mods but from my experience I will give my 02 cents.which of these mods that you listed were just put on from last time you blew her up or were they all on from last blow as well.? were they on before the first Blow?
what type porting was done MX,TT(low end torque or topend power) or rec riding ? and by who?
Looks like you have stock 34MM carb on it,(got to go)
if you ported put rad valve etc, you have to rejet for sure
K&N clamp on or in factory airbox? clamp on filter big difference need to rejet K&N in box not much diff prob no rejet.
FMF got to go (junk) I had one FMF gold series pipe when I tried Paul Turner pipe really woke up my quad! better Mid to top power FMF was good for low to Mid not top lol compared to Paul Turner pipe.
hopefully with all the great help on this site we can get her figured out for you.
Casey

 

Was the porting done between the last blow up and this one? If it was and you did not rejet, then you were definitely running lean. Remember lean mixtures make better power than rich ones, that's why people always say "It was running great till it went kaboom!"

 

Yes, the porting was just done.

If it were lean wouldn't the top of the piston and the plug show this though? The top of the piston had a nice tan color and the plug was a nice tan color also.

 

It has been my experience that there should be .025 to .030 thousands clearance between the piston and power valve closed.

 

can you post pics of the plug and piston in your bio. Yes tan is perfect or borderline lean. depends I have to see it? if its tan when you are lugging it then its probably lean when your really on it. you are supposed to warm it up normally then do plug test to see if jetting is correct.The way you do plug test is find area that you can go through all the gears wide open then kill motor and coast to stop in nuetral or clutch in and check color of plug. you said you were lugging it in 6th at 1/4 throttle which should be running rich not tan. I bet its lean bro. Did you get a chance to check impelor on water pump. or Power valve?what about porting what kind of porting you didnt say?did you have head machined for higher compresion?what kind of gas are you running Octane level?

 

You said you died when you were at 1/4 throttle and that your plug showed a tan color. That means when your at 1/4 throttle you have a pretty good mixture but i bet when its wide open shes running lean. To do a correct plug check you have to go though all the gears and then kill it when your wide open and coast to a stop and then check your plug.

 

This may noy relate but a friend of mine had his 250r melt down 3 times from a base gasket failure. I think it had something to do with the sleeve install made things not line up right.

 

I am told that the way the piston and cylinder looks something must have entered the cylinder and tore things up. There was a cut in the side of the piston and in the cylinder about 1 1/2". The cut in the piston was about a 1/16" deep.

Anybody ever have this happen??

 

both your washers there from wrist pin?

 

How do your rings look? One of them may have gotten caught on a port if they were not chamfered after boring.