The gas tank was leaking tiny bits of gasoline. I didn’t notice for a very long time as it was sucked up by a rubber block. At some point I noticed that the rubber block was twice as big as it is suppost to be and over some time very small amounts of gasoline dripped to the ground. The source for the leak was on the bottom of the gas tank, where the brackets for the antivibration rubber blokcks are welded to the gas tank. Obviously the metal of the gas tank was weakend by the welding process so it leaked after about 50 years later.
So I went to the a welder company and they laser welded around both brackets without even damaging the paint. It’s now sealed again and probably good for many more years.