It's very possible that's not the correct gas cap, and it's not sealing. That would let vapor escape and water in. We bought a similar one off of ebay, and I'm pretty sure it fit better than that.
There should be a small filter inside the fuel line right where it connects to the carb. It's not great, but it's something.
I would check the battery voltage. Just because it's a new battery doesn't mean it's fully charged. Often they are not.
Before you pull the carbs, I would drain the tank, check the condition inside, and refill with fresh gas.
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