Blaster has blown up twice now, please help!

[BlasterBlastin]

Hello, i recently purchased an 03 blaster with airbox lid delete, stock expansion pipe, fmf slip on, highflo airfilter.
i ran the quad for about 3-4 hours some long trips one fouled plug.
purchased an FMF gold series expansion pipe, used, the kind with the bolt on flange, and spring to hold pipe in place. i then welded a loop on the flange for the spring and installed with gasket maker and it all seemed great; no leaks, we pulled apart the carb and found a 340 jet, FMF recccomends a 270 for my mods..? i went down to a 300(seemed to still be safe) as recccomended by a friend, re tuned carb, seemed to run alot better, more responsive. lasted maybe 45 mins before it died on me and lost all compression. figured, leaned out, will fix it and go back to 340.......... and go up If it wasnt rich during break in..

next i purchased a knockoff topend and jug kit from ebay, as it had good reviews, installed it all, went back up to a 340, deleted oil injection, mixed at 20:1
followed a breakin procedure on here, went great, pulled plug, seen this.
sent it off to a few friends and compared with charts, everybody thought rich.. ( keep in mind i didnt go past 3/4 throttle due to break in )

then i went out riding again, and about 45 mins into the ride it blew up AGAIN.
I ordered a full vertex gasket kit to do crank seals and EVERY gasket, and a topend ( on warranty ) and will be leakdown testing before first run... ANY tips or leads? recccomendations?Should i proceed to plug chop provided the leakdown test is all clear? should i up jetting from 340? im located in sw ontario and the altitude is around +190 sealevel.
temps were around 2c the first blow up and second was around -15. detonation(pistonhole) was 2nd time.
i tryed provide as much info as possible.
thanks for your time it is very much appreciated.

2nd time

1st time

1st time cylinder

 

[Tooloose]

Nice place to make a Rat hole in top of the piston don’t you think??? Mixture, mixture, mixture is way off. And or timing is way off , or air leak (vacuum )leak . But a vacuum leak down tester and test before ruining another piston !

 

[RIDE-RED350r]

Yes, a hole hammered into the piston crown is detonation. Ignition timing or too high of compression ratio for the fuel you are running will cause this.

Lean condition damage will present itself as a piston with melted away material at the very top to just below the ring lands predominantly on the exhaust side of the piston. Will also show aluminum piston material smeared and stuck to the exhaust side of the cylinder wall.

Whenever an engine fails in ways like this, it's not enough to replace the top end and call it good. One should find the cause of the failure and address it or else you will be doing it again

 

[Tooloose]

Yes detonating/or spark knock will destroy your top end! Low octane fuel is a culprit to cause this along with too high of compression. Increased Timing will cause this issue, For each degree of timing causes raised cylinder temperatures. Approximately 900 degrees cylinder temperature is a normal range give or take a few degrees. Plug chopping is a real good example of how to judge whether you’re too lean and or too rich. Hope this helps you find the perfect balance