There was something on the bottom that wasn’t supposed to be there! Looked like cereal. What had the previous owner done? The stuff crumbled, and disintegrated easily between my fingers. And it became powder-like when it dried. I removed all (I thought) by pumping the gas out of the tank filtering it until no more of the stuff showed up. There was LOTS of it.

Unfortunately, it did not help. Same thing happened, the engine stopped abruptly after 10 minutes of running it. I looked in the tank again and found more of the stuff. Filtered the gas again until there was no more of the unwanted cereal. Repeated this SEVERAL times but with same frustrating result each time. Where did the stuff come from and what was it? And why did the engine stop so suddenly and why did I not find anything in the gas filter?
I finally discovered that there is a built-in (not replaceable) filter in the vertical pipe/tube INSIDE the gas tank. The filter is difficult to discover since it is about 15 centimeters or so up the tube.
The filter and its location explains why the engine stopped: after 10-15 minutes of running the engine more and more cereal is sucked into the tube and is stopped by the filter until the filter is entirely clogged and no more gas can pass through stopping the engine. After a while the (dirt) sinks to the bottom of the tank and the engine can be started and run for a short while.

I removed the filter by drilling thru it with a long drill. Now the engine ran fine!

But since I still had the problem with the cereal and didn’t want it in the engine I decided to pull the old original (1979) aluminum gas tank. To reach the tank, the floor had to be sacrificed. I was about to get an unpleasant surprise.Also, the carpet was in really bad shape and replacing the gas tank meant sawing up a large hole in the floor so something needed to be done about the floor anyway