I have found a reply on another site that answers my question:
"I disagree with the gist of this thread which is that out-of-spec valves display immediate symptoms. Valves that are anything but radically out of spec will have the same immediate symptoms as not changing your oil - NONE - the bike will run perfectly fine.
As valve clearances shrink (the typical result of valve wear), but not so much minimal clearance goes away, the bike will run better as valve lift is increased. But once all clearance at operating temp is lost, katy-bar-the door as you'll start losing compression and risk overheating a valve, at best leading to hard starting and poor running, but at worst, catastrophic valve failure. There's little problem with tight valves until they get too tight, then all hell breaks loose.
As valve clearances grow (which is not typical but does happen), the bike will gradually lose power (but probably not enough to notice) and the valve gear might be a bit noisier. Valve train wear will increase somewhat as now the valve is being "slapped" open rather than brought off the seat gradually with the opening/closing ramps. But all these things are gradual and won't show up for quite some time.
Bottom line is that there are VERY FEW SYMPTOMS OF IMPROPER CLEARANCE. That's why valve maintenance - like oil changes - is preventive maintenance to reduce the risk of valve failure and to insure normal valve life. It is not something you should do after symptoms appear. And, like oil changes, you check the valves at some mileage compromise between the maintenance expense and the risk. Unless you have better data or some kind of intuition that the mfg doesn't, it is probably best to follow their recommendations, more or less.
It always amaze me that so many owners are perfectly happy ignoring the mfg's valve check intervals because they hear on the internet that "they never need adjusting", but wouldn't think about not changing their oil 2x or 3x more often than the mfg's recommendations. This makes zero sense.
- Mark"
The answer can found at http://advrider.com/index.php?threads/symptoms-of-valves-out-of-spec.627694/
