Don't really know how to explain it, but say you put your finger on the breather hole in the air intake, would you feel any vacuum on your finger? Like does the air rushing into the engine create positvie pressure in the air intake (like on a boosted car) or does it create a vacuum (like if you put your vacuum cleaner against your hand)? What about in the intake manfiold plenum/runners?

 

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The motor creates a vacuum. Its always making vacuum. Buy a vacuum gauge and check it out.