Grenaded Engine

[bmw633]

Hi,

I bought a 2001 GZ250 with 10,000 miles on it for my 25 year old daughter to learn to ride on the road. I removed and cleaned out the carburetor, put a new spark plug in and changed the oil and filter, and 500 miles later, she was on the interstate going around 75mph when the engine made a noise and stopped running.

The head was pulled off and what happened was one of the exhaust valves broke off and embedded itself in the piston, causing catastrophic damage to the engine. I found a junkyard engine with 4500 miles on EBay, so my question is, what caused the exhaust valve to break off from the stem? Is it a manufacturer defect? I have read about other GZ250's losing their exhaust valves, so is this common with this bike?

The carb was clean as a whistle. She was running 87 Octane gas, not the 93 octane high test stuff. Could a lean situation caused the valve to get overheated?

Seth Anderson
Moon, VA