Well, it sounds like a vertical shaft engine on a walk behind mower. RPMs set depends on blade tip speed. Typically, a 21" mower is set at 3150 / 3050 on a 22" which allows for enough fluctuation to remain under the max BTS as the mower is doing its job. The plastic breather typically has a check valve at the bottom. It is common to replace these kits for leakage. You have to also be careful not to tip the mower over too far or upside down as these breathers turn into little oil pumps.