Spark plugs don't "dry out" with unleaded gas like they used to with leaded gas. It's possible they're not sparking under pressure. Maybe swap the left and right plugs if you don't have new plugs.
Not sure if you realize it but there is a three spark plug caddie under the seat.
It's a black plastic bottom with a clear top. ( another part not to break
)
When you remove the carbs, make sure you pull/twist them straight out. There is a phenolic ( plastic) heat shield insert that can break.
With the rubber grommet, either put the old one back in or find a similar solid plug at your local hardware/automotive store and put a hole in it.