That hose would originally have been connected to a port on one of the SU
carbs. Dirty air ventilated from the crankcase would join the air from the
air filter and would be fed into the engine, so any oil in the ventilated
air would burn in the cylinder rather than going straight out to the
The need for this is because a bit of the pressure from combustion gets past
the piston rings and into the engine block (blow-by). This would pressurise
the whole engine block, so the pressure needs to vent somewhere. If the
vent is blocked the most common result is that the pressure forces the
dipstick out, and oil dribbles from the dipstick hole.
Some Spit owners who have converted to webers have simply fed the pipe from
the vent into an open can supported somewhere in the engine bay - the can
catches the oil drips and is emptied occasionally. I believe you can also
acquire a ventilated oil cap that allows the crankcase to breath without
letting (too much) oil out. There may be other, more sophisticated
solutions out there too...
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