lapping the valve worked. With a screw driver, I applied a small amount of lapping compound, about half the size of a pea. I ran the drill at low speed in both directions as Roland suggested. Then I put a series of paper towels soaked with WD40 between the valve and the seat in an effort to collect debris. I ran the drill between some of the cleaning cycles. The valve spun smoothly in its seat at the end. The combustion chamber is packed with rope through the spark plug hole to hold the valve up and it may have contained some of the debris. There could not have been much that was causing the problem. With the spring reinstalled, it now holds a vacuum like the other intake runners.
