I believe your '82 has a totally different type of powervalve. It should have the older cylindrical powervalve which has a lot more surface area and two seals on each side(o-ring and a seal). If you have never had it apart, and there is no oil built up under its' covers, chances are it is(and has been) stuck. The carbon builds up and won't allow anything to pass out of it.
Your '90 has a different setup and may have bad seals for the powervalve.
As far as dissasembly, cleanout and re-assembly, get a manual for your bike and read it. We could give you the idea but I couldn't provide you with details or pictures like the manual can.