Why the 89 octane instead of 87?
Fewer people buy 89 (or higher) octane rated gasoline than 87 octane. Therefore, you might be at greater risk for buying some water along with the gasoline. The high turnover (storage tanks emptied and refilled from tanker trucks) for 87 octane means that the 87 octane is probably fresher, still has most of its additives, and has had less time to absorb moisture from the atmosphere.
The 89 octane by comparison moves much slower in a lot of fuel stations, so it is less fresh, has lost some of its goodies, and may have more moisture content.
Ethanol, while being somewhat a water dispersant, is nonetheless a "water attractant" too.
The water gets attracted to the "emulsion" of fuel and alcohol under certain conditions, only to be precipitated out later.
The precipitated water can collect on the bottom of the fuel tank or the bottom of the float bowl (or both).
This blob of water rolling around in the float bowl can be drawn into the jets of the carb sometimes (think....agitation) and will plug up the jets momentarily because the thicker, more viscous water takes a bit of time to be drawn through the jets.
Remove the bowl and check for water. If you find blobs of water, you really should drain the tank and refill it with all new fuel.
The gas you drain from the tank could be carefully transferred to your car tank, just be careful you don't let the water go into the funnel too.

Tony: Mownie makes good points , I would just add that you also pick up some form of fuel cleaner / conditioner to add to the dry gas within a fresh tank of 87 octane fuel . It may be other debris within the fuel system along with water contributing to your problems , a new spark plug along with the fuel filter that you have changed recently would be a adder for extra measure of assurance at this time .

I wouldn't discount the possibility of the seat switch working "marginally".
Level ground may be fine, but a few bumps and your weight shifts slightly.....
You might check the adjustment.

Upon closer reading, I doubt it's the seat switch, since fuel is "dumping".