Piston Crown Melted?

[Squeeze Play]

I have a 1998 SRX 700 with trail porting, shaved heads and shaved skirts. I am jetted very conservatively and retain the stock airbox with baffle board. I just noticed when I pulled my powervalves, in cylinder 3 (mag side), the piston crown is melted at the exhaust port side. It did not affect the rings thankfully, but anyone have any idea what could cause this? I did run an MBRP exhaust can last year and it ran like crap, so I put the stock can back on. I wonder if low back pressure from the can or bad gas may be to blame?
Help?

 

[98srx6]

can you get pictures of the pistons and heads?

 

[Squeeze Play]

I wish I had time to pull the motor apart myself, but I have a 17 month old child at home. I usually have my work done with a local yamaha tuner. I have not talked with him yet, but I hope to get many opinions so I dont do this again. The melted spot is about an inch long and looks like a waterfall. It is almost touching the ring and it looks like it was not thick enough to touch the cylinder wall. Thanks

 

[Turk]

Been der dun dat....you were lean even tho jetting seems conservative. Those mods make the engine want more fuel. 1 day with a very high barometer &you will go boom!

 

[Wilson]

Turk is right fatten it up I have melted enough pistons to know .First cold day and you are toast .Bigger jets will save you big $

 

[srx74]

Too Lean. Do you recall what size pilots, Mains and air jets were conservative.

 

[Squeeze Play]

Thanks everyone, I will replace the piston and fatten her up.