Art,
Here is the picture of the cupping procedure,
You don't want to over do it. I did this to one valve that had a petal that wasn't sitting flat. I put about 1/32" bend in it, and it worked well. This seemed to work better than using a second cut down petal valve over the main valve.
I was going through all my pulse jet stuff, and most of it is based on performance increases. If you have any interest in this type of stuff I can start posting it.
Most of the information I have is either from Bruce Tharp, or Mike Hazel. Mike use to sell what he called a jet pack that was about 1" of reprints of pulse jet articles. Lot of good information there.
Greg