This all has been a very interesting topic and read it front to back. It seems topical and resurges in interest often with different makes and models. Some years back I worked a spell on a bike backlog for a local shop and they carried a number of bikes modern and vintage. The owner was very interested in the BUNN Breather issue for some of the modern Enfields as we built hot rodded versions for some customers. People tend to get tunnel vision and not look beyond their specific make and model. At that time I amassed a huge amount of discussions and tech materials from a wide range of sources and shared it with the brain trust at that shop.
We concluded that the BUNN breather had merits but anyone could make their own versions and I suspect thats what did in the supplier.
This topic is hotly debated on Norton forums and seems to be several different guys supplying their perfect mousetrap for Nortons. Some of them are quite clever and well engineered albeit expensive however some just modify and adapt a japanese bike metal PCV flapper valve and that seems to work for a fraction of cost.
I used to build some V8 performance engines and we installed a few with a vacuum pump setup that worked really well, Eliminated leaks, increased ring sealing and slight boost in power. You can make your own but Moroso sells kits. Not totally feasible on a A10 but the concepts are worthy of study for a better understanding.
I have some HD bikes and multiple Buells, As universally known HDs are known themselves for leaking oil. Buells design for their breather system is I believe the final word on solving these issues in reasonable way. The sportster engine design ran a number of case breather setups as well as eventually having breathers thru the cylinder heads with "Breather Bolts" that vented INTO the air cleaner which created a slight vacuum as well as met EPA emissions requirements.
But Buell changed that system into a baffled breather system out the TOP of the cylinder heads with PCV valves and hoses. (Again culminating into the intake) Many people in the Buell world have setup alternatives where they go to a check valved catch can, or vent openly but most agree the top of the head PCV valve setup works really well. Many other V Twin models now copy this design as well or adapt this system.
I have greatly angered some BSA owners in the past arguing the timed disc setups in BSA Singles and twins is inadequate (FACTORY STOCK IS BEST you heretic!) But simple testing shows thats not true. I do think on a BSA Preunit twin that the venting from the case UP thru the head is problematic but could be addressed easily enough but likely would be visible externally. It seems logically that 2 large fittings (one at the head and one off the case) into a good PCV-check valve set up would be the way to go.
I am not going to tell you guys what you have to do, but instead encourage you all to look at what other makes and models do like I did and adapt what works best for your situation,.
