Sorry, I just now noticed this thread. Leaving the fuel valve open didn't cause any problems, the fuel left in the bowls is what caused the problem. The carbs will have to be removed and cleaned and I would also change out the sparkle plugs while you're at it.
Before you do anything about getting it started CHECK YOUR OIL the needle and seats in the bowls may have allowed feul to seep and you may have feul in your oil smell it if you detect gas smell change the oil then move on. I leave my lawn tractor and snow blower setting with feul open a lot longer and never have an issue. I would change the plugs and add fresh feul, your carbs shouldnt be craped in 7 months. After you run the old feul you can run some qaulity feul system cleaner.