Just barely too lean. Piston got too hot, melted over the top ring at the exhaust jamming the ring till it broke at hi rpm. One more jet would have got it. Or a forged piston could have handled it. These cheap cast pistons just cant handle long high rpm runs unless you tune it down, and that is the opposite of what we do. I found a 47.5mm Yamaha piston that works in the Malossi. Just gotta hone out .5mm. I get the piston for $8.00 from Japan. It's a bit taller/longer so a base spacer or stroker is needed, but it is the build I am working on now. I'll keep you filled in. Ps The piston is from an out of business shop liquidating his stock...14 pistons left.