Is fuel/water coming out of that drain hose, when you loosen the screw?
If not it may be clogged. Blow (gently) into those to ventlines on top of the carb. It will force mud and water out of the bowl.
Turn the gas off and let the bowl drain until it stops. Shu tthe drain off and turn the gas back on to refill the bowl, for 20-30 secs. Turn it off again and repeat the flushing. That should take care of it.