The carburetor needs to clear excess fuel and crap from time to time due to rough conditions or temperature changes like splashing through a puddle. It can also cause a vacuum in the carb during expansion.
The discharges flows out and down towards those hoses underneath your bike. In the right conditions, your overflow will try to suck air and if both hoses are in a puddle, water will go up the hose into your carb.
Air is lighter than water so when you re-route and the overflow has a vacuum, the water stays in the lower tube.
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