When you shut off a small generator by killing the spark, you’re still drawing in air and fuel at 3,600 rpm’s. You’re loading your cylinder with unburnt fuel, which does no favors to your cylinder wall lubrication. Imagine revving your car or truck to 3,600 rpm’s, every time you shut it off, and simply killing the spark plugs while still taking in raw fuel.
All modern engines, gasoline and diesel, stop injecting fuel when you shut down, but these little carburetor engines keep drawing in fuel. Not ideal.
I’ve got a Honda GX engine since new, and initially I shut it off with spark. After a few weeks of no use, it would always lean speed surge for the first 5 minutes or so, sometimes almost stalling. Probably jetted lean from the factory. Once I started shutting it down by the fuel shutoff, It’s started up first pull, and immediately steady speed. Can argue with that