I have often had this exact symptom on an old beast of a mower. It was also a Tecumseh, but not sure the engine brand really matters. In my case, it was water in the fuel which settled to the bottom of the carburetor float bowl. I could do a quick fix by just loosening the nut fitting on the bottom of the bowl enough to let the water and then some gas drain out. Water is heavier than gas so it sinks below the gas. Usually that was enough to solve the problem. A better fix is to remove the carb and clean it, and make sure that the tank is as clean as you can get it too.
Yes, fuel with alcohol is a problem, probably explains why I had this problem once or twice a year.
Our local mower repair shop will do a quick turn-around on carb cleaning / rebuild if I bring in the carb removed from the mower. In busy season, I get the carb back the next day. If they're slow, I can pick it up in the afternoon. Maybe that's just because they like me, it's certainly not because I'm a big spender! They replace the needle and gaskets, and use an ultrasound device to clean out the carb internal plumbing.
That said, there's a spring on the carb on my machine that I once managed to attach incorrectly, which causes the very same problem. I can't describe the spring, but a good repair shop will show you how to connect the springs and linkages on your machine.
I'll bet you have one or the other of these issues.
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