Crankcase breather / ventilation hose. It should go back to the carbs or
intake manifold depending on your setup. This is also the location of PCV
(Positive Crankcase Ventilation) valves. If you plug it, you could
pressurize the crankcase and you'll find you have oil leaks where you
never thought possible. Vented, it just relieves the pressure and vents a
bit of oily vapor. The best place is to route it back to your intake
where the vapor can be burned with the fuel.
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