Was cleaning my air filter recently and noticed a thin layer of dust accumulated on the plastic ducting. The way I see it, some dust is always going to get in, as SC pointed out (at an inappropriate time). After all, if air gets in, some fine dust must also get through the filter. This led me to believe that the dust at velocity acts like a mini sand blaster, removing what I have been led to believe is a titanium coating on the valves. Maybe once this is removed, then the valves start playing up. Of course, the exhausts wouldn't be subject to the same abuse and are spared the humiliation. This could explain why street machines such as R1's with similar revs don't chew valves cos they're not sucking dust. This would also explain why CRF's are having a worse case of it cos they're always sucking the WR's dust. I'm more or less convinced that dust is the prime cause of failure combined with modern valve material, but I can be swayed. The moral is "always ride up the front"