What are the acceptable clearance ranges?
Acceptable clearance ranges are:
I think somewhere else in the manual it says that the intakes are 0.10mm and exhaust 0.13mm. Anything in the range listed is probably okay, but make sure to set them to a consistent number. The valve clearances are the same for all 250 Ninjas.
Are there any advantages to selecting a specific setting within that range?
The valve clearances affect valve timing and overlap (the period that both intake and exhaust valves are open). With everything stock on the bike, the maximum amount of air gets trapped in the cylinder at about 9 grand (peak torque).
Before 9 grand, the overlap actually lets a little air go backwards through the intake valve before it closes, reducing the amount for combustion.
After 9 grand, the valves are opening and closing too fast, so less air goes in per cycle.
One way to shift this slightly is with the valve clearance. If the clearance is at the minimum, the overlap will be more, and the valves will open more. This shifts peak torque (and therefore peak horsepower) up slightly, at the cost of low to midrange performance.
If the clearance is at the maximum, the overlap is less, so it will trap the most air in the cylinder at a lower rpm, boosting the low to midrange, but at the cost of the higher rpms (because the valves aren't opening as far). Having the valve clearance at its maximum also allows the valves to stay in contact with the valve seat longer (tranferring heat during that time), and helps to prevent burning an exhaust valve. It will idle smoother, because there is less overlap that allows exhaust gases to go into the intake, and lean out the mixture for the next cycle. This is why an engine with cams that put peak torque at 9000 won't idle smooth at 500 rpm.
The exhaust system used also plays into this, because it determines the amount of time the exhaust valve can be open before the gases start flowing backwards. If you change the exhaust, it changes the rpm at which gases will reverse, and let exhaust gases back into the cylinder. AR (anti-reversion) exhaust systems are supposed to give the broadest range of power, because the can prevent reverse flow at a lower rpm.
Anyway, tighter clearances will increase top-end power, and the chance of burning valves, reduce low-end power, and adversely affect idling.
Looser clearances will make it idle better, increase low to mid power, and help to prevent burnt valves, at the cost of top end power. Of course if you go too loose then you will have a lot of valve train noise.
You can always try both extremes and see which works best for the kind of riding you do, and the exhaust system you have.
What happens if the clearances are too tight/loose?
Generally, running them too tight will increase top-end power for a while until the valves overheat. Then the valve and seat will erode, allow the valve to start to tulip, which will close the clearance even more.
Running them toward the loose end of their tolerance will normally increase low-end power (at the cost of top end), and make the valve train noise louder.
One of the most important things (aside from staying within the tolerances) is to set all the intake valves clearances as close as possible to one another, and do the same with the exhausts. This, just like syncronizing carbs, has a big effect on how smooth the engine runs, especially at low speeds. Without going into great detail, this is because the carb sync only determines that the intake valves have the same pressure upsteam, but the timing point at which the valve opens and closes determines how much of that air will get into the cylinder. These combined ensure each cylinder is doing equal work, and therefore run smoother and more efficiently.
This procedure is something that you should really learn to do yourself. Nobody will pay attention to detail (or be more concerned about scratching your bike) than you. If you have a friend that knows how to do this, seek help for the first time (preferably have him demonstrate on his own bike first, and see if it still runs afterwards).
After valve adjustment, resync the carbs, because if the valves weren't set correctly then the airflow may change slightly, requiring the carbs to be rebalanced.