Get yourself a Haynes/Chilton manual, it will tell you how to adjust the valves. Adjusting while it is running is something you do on some pushrod motors, but not on the L series. You'll need a feeler gauge and some wrenches, I think its a 14 and a 19, but it's been a while. It's not hard to do and is thoroughly explained in the auto parts store manuals. A good L series valve cover gasket can be taken off and reinstalled many times before it tears or anything. If you have a cork gasket on there you better get a new one from the auto parts store while you're getting your wrenches, feeler gauges, and manual.