There is a lapping compound called Clover
http://www.newmantools.com/clover.htm I have been using it for 50 years. I am no longer able to get it locally but it can still be bought on the internet. However, care must be taken that you do not get the seats too wide. I have valve grinding equipment and use lapping as a finishing touch. Also, your valve guides should be measured as well as your valve stems. This takes a mike and a small hole gauge. If you know an old timer who still has the equipment, possibly you can get done reasonably. Black sooty plugs can be rich mixture or from oil consumption. A lean mixture is more likely to burn valves than a rich mixture.
