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Presumably with the cap removed and engine running then the air in the crankcase beneath the pistons has to go somewhere - usually out of the engine breather but like electricity it will take shortest path if the cap is off.
Interesting aside a single cylinder configuration can never produce as much power as an inline 4 because of pumping losses.
In a 4 the crankcase pressure stays pretty much the same, because as one piston descends another rises by the same distance so the pressure beneath the piston is pretty much constant. A single has to displace its swept volume of air on the power and exhaust strokes.
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