Problem solved, other problems also solved
Found the problem: sludge in the gas tank. I had been checking the fuel supply at the carburetor, and at the fuel pump, and just below the fuel filter (which I had replaced twice already). I SHOULD have checked the fuel supply as it came directly out the tank. There was just enough sludge to clog the exit hole. I'm going to put a strainer in it:
https://www.steinertractor.com/ABC1...HLDL5qwwHV2oUzQ6zJqfEWT07ZZu_DEhoCfjUQAvD_BwE
@mechanic mark, thanks, I forgot to mention I also replaced the air filter. I also replaced both coils before I discovered the fuel tank sludge issue. Not a waste of money I think because they were super rusty, and the clearance to the magnet on the flywheel was way off, like 1/8". Good opportunity to clean the rust off the magnet. Apparently, a business card is the proper gauge to use between the coil and the magnet.
I'll describe the rest of my story in case it helps anyone else:
Last summer I noticed rust in the gas tank, so I took it off and cleaned it out and put in phosphoric acid. I dried it out for a few days. Then I put in a gas tank lining paint. I forget which one but it was a well reviewed product. Shook it all around in there and let it sit for a few days. Word to the wise: this is not enough time! There are baffles inside the tank, which are there to collect puddles of gas so when the machine is on an incline it won't stall. These baffles make the thing nigh impossible to drain completely, even using compressed air. The best way I found is to set up some kind of fan to blow air into it for a few days (like a hair dryer, or the exhaust side of a shop vac). Unless someone else here has a better way.
I discovered this because, after a few days' dry time, I
thought the paint was all dry, so I put the tank back on and filled it up. Started sludging up fuel filters one after the other, very quickly it was apparent that the paint wasn't really dry at all. So I drained the tank out again and set it up with a hair dryer for a week.
After this, the machine ran for the rest of the season. Cleaned the carburetor bowl at one point but did not clean the rest of the carburetor.
This spring, I had a sudden loss of power, and noticed oil coming out of the breather valve. Turned out that one of the bolts that holds the rocker for the valves had stripped out. Fixed that using a heli-coil, piece of cake.
But then, the machine kept stalling after running for an hour or so. That's when I cleaned out and rebuilt the carburetor, because a friend said it was probably a stuck float. The carburetor did need cleaning, but of course it didn't solve the problem.
Here's another story, about a totally different issue that happened right after I finally found the sludge in the fuel tank: there is a hydraulic fitting on this machine that tends to come loose, and which I had to grind a wrench real thin in order to tighten up, nothing would fit in the tiny clearance. Ok, problem fixed. But, it had been leaking all over the winter, and I was ignoring it until spring. Oil got all over the deck and polluted the belts. Now they smoke until the oil burns off enough to create friction. I need to clean the deck some more. Cleaned the deck with soap and water and in the process found a belt that was worn out in one spot real bad, so I replaced it. Had to take a different belt off in order to replace the first belt, and when I put that one back on I missed one of the guides. That belt was destroyed very quickly. Had to order a special Hustler belt (it's somewhere around 49 1/2", not a common size). In the mean time I put on a 49" belt so I could mow, yes the blades are always engaged now. Got that put on and went to start it up: oil light came on but the motor wouldn't turn over! Tried jumping the battery, still no. Bought a new battery; still no.
Sat there and cried for a few minutes, and then dried my tears and started examining every wire on the damn thing. Turns out: there are two fuses which are inline with the electric cables that go to the starter. One of these had blown.
About those inline fuses: there is a little tubular plastic case that looks just like a regular clip but it also happens to contain a fuse. Inside the case the end of each wire terminates in a little cup-shaped crimp fitting, that is used to make contact with the fuse when the case is assembled. I think there's a spring in there too. The copper strand is prone to breaking where these are crimped on, as I noticed one was halfway broke when I finally discoved the location of the fuses. Just another spot to check if one is having electrical issues. I'm going to have to find out where I can get these little crimpy guys because I need to replace at least one before the wire breaks.
There is another mystery I discovered during this process which is a two-pole button switch that is clamped permanently in the closed position inline with two (ignition?) wires. I have no idea why that switch is there, but it is another place I might suspect if I was having electrical issues. I wonder why they put that in there?
