The above plumbing is described in various threads on the forum. Basically it should be arranged so the crankcase pressure with oil vapours should vent to the carburettor after the air cleaner not through it and burnt in the engine. Clean replacement air should be drawn in through the other vent after filtration by the air cleaner. The result of this should create a slight vacuum in the crankcase. On an SD1 the larger piping goes to the carb body and the smaller to the air cleaner. Both pipes have suitable flame traps in them.
