Practical valve adjustment interval is 3000, 7000, and 11000 miles, because the exhaust valve will slowly close until it's settled in, and then rarely move at all. The intake valve rarely moves anyway, because the heat is on the exhaust.
Since you can take the plastic off, you would be better served to buy a good socket set from Home Depot and at least inspect the valves, and adjust them if they need it.
Unless your engine is hard to start or dies during idle, the valves may be fine, but you do need to inspect and measure, so even if the mechanic inspects, and finds them to both be at .004 inch, he won't adjust them, he'll just button it up and bill you.
The 250 is so much easier than the 500, there's a lot more room to work, and only 4 bolts holding the valve cover on, so open the flywheel port, the sighting port, line up the centering mark, make sure both valves are loose, then slip a feeler guage in for a .004 inch gap top and bottom, if the bottom won't accept it, adjust it so it will, and yer good forr another few thousand miles, and next time, like Rayman says, you'll be a pro.
