At my neighbor's advice I added a fuel system dryer, Isopropyl Alcohol, and the problem is solved, it runs great again. So, it was water in the tank.
On a second note, I used to run an electric fuel pump but changed to mechanical years ago. Beats me why, it was too long ago. If I let the car stand for more than two weeks it appears fuel starved and starts hard, so I pour a little gas in each Weber carb and it fires up immediately. An electric pump has been ordered and will be managed by the existing fuel pressure regulator.