Vented rocker covers

It doesn’t

 

My understanding is
If it’s a good engine with good piston rings the stock crank case breathing only struggles if you increase the bore size and/or run at higher revs does

 

I read it somewhere and Zed pointed it out when he had his shop. Dunno why but a breather sorted it.

 

If you can make the engine rev faster with twin carbs, the crankcase pressure will pulse harder as pistons push the around faster .it should not leak more. A bigger breather may help reduce the pressure pulses.