Try emptying your tank onto a hard surface then put a match to whatever comes out. If it burns then its fuel. If it doesn't then its water!
Seriously though, it could be water in the fuel, but you would have to drain the tank and carbs and refill with clean petrol. Its the only way to tell unless you can take the fuel pipe off and decant some into a clear container. The water will be nearer the bottom.
It may also be an ignition problem such as the cdi unit, plug leads or plug caps. Failing all this it may be the cam chain has snapped (????????) which may or may not give intermittent compression resulting in what you are getting. Probably not this but then who knows
My advice would be to start simple, then panic
