Dropping valves

[mtbdrew]

So beside the chip were the parts compatible as advertised? Did you order the cylinder head assy and cylinder jug with piston?

For some background, my youngest son bought a 2002 GZ250 and the second week in the bike stopped running on the way home. After taking the cylinder head assy off found the two exhaust valves were missing their heads. Looks like the was a detonation in the exhaust port the force of which had embedded parts of the valve head into the piston top and shredded the upper surface of the bottom half of the cylinder head. Spark plug could not be removed. The front nut between the exhaust pipes was also missing and part of the gasses from the detonation had escaped between the two halves of the head assy. This completely blackened the screw stem and melted the plastic around the started cable.
Drained the oil and found no metal filings or bits. Rest of the motor now turns freely so hoping all is okay. Bike didn't come to a screeching stop and the piston was not seized in the jug. Have parts on order form Chinese supplier on eBay with same comment that should work.
Would really like to know what cause the detonation. Son said the bike was backfiring when he turned it off. The carb main adjust screw is turned all the way in which doesn't seem right to me. previous owner was running with over 2.5 quarts of oil and there was about another 0.5 in the breather box.

Andrew