As to how a valve wears; the seats are a hard steel - the valves are hard, but not as hard as the seats - without the cushion that leaded fuel provided, the constant contact between the valve and the seat wears both, but the valve being softer than the seat, the valve wears fastest. The intake valves get absolutely no cushion - the exhaust valves run hotter, but are smaller and get carbon and other byproducts of combustion to contaminate their faces and seats, therefore they do not wear as fast as the intake valves. The life of the valve can be extended through the use of fuel additives (such as oil and lead substitutes), but the additives also raise octane. If you are running the recommended octane fuel and use additives, you should compensate for the change in octane proportionately. If the additive raises octane 2 points, use fuel 2 points lower that the recommended octane - using fuel more than 3 to 5 octane points higher than recommended by the engine manufacturer will result in a loss of power, lower fuel economy, higher exhaust temperatures and accelerated engine wear.
My CRF had Titanium intake valves - the valves were light, but wore out very quickly - I did not ride as hard or as often as some other CRF owner's (I rode most every Sunday for about 2 hours in the woods) and I stretched the life of my first set of intake valves out almost 2 years - for the guys that rode every day for about an hour a day on an MX track, they had to replace their valves once a year.
While I got away with adjusting my valves 2 or 3 times a year, the frequent riders had to adjust their valves every one or two months.
It's pretty easy to know when the valves need adjusted - when the engine becomes harder to start than normal, adjust the valves. For the guys kick starting; if the cold engine usually started in 2 to 5 kicks and got to where it had to be kicked more than 8 to 10 times, adjusting the valves would get it back to starting in 2 to 5 kicks.
Is this as clear as mud? If you want more info or a clearer explanation, search valve wear and octane articles published by scholars specializing in those subjects. I'm just a layman mechanic.
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