At least for the '72 and '73 750s that have the drive side rear crank case breather, the CNW reed valve acts as a secondary scavenging pump; it acts as a positive return of oil to the tank that for whatever reason is not scavenged in the first instance. That fact distinguishes it in function from any and all of the modifications that locate a breather take off on the timing case.
Kudos too to Old Britts for bringing the fact of lost oil pressure due to the '72/'73 design to the attention of the Norton world.
Saturday I spent a few hours fabricating an adapter plate to mount the business half of an XS650 reed valve onto the back of my '73 750's drive side crank case.
It wasn't simple or easy. After removing the stock "L" breather casting from the crank case, I commenced to make the adapter plate from a piece of steel (about 3" by 2" by 3/8" thick) by drilling holes that aligned with the breather mounting bolts, the reed valve center and the reed valve's own cover plates holes. The cover plate, that's the side for the reed valve's inlet, the one with the larger 5/8" beaded edge tube on it, got abandoned and substituted by the new adapter plate.
I also had to weld up a U shaped bridge to go over the outer casing of the reed valve and through which the mounting bolts pass. The sides of the reed valve housing also had to be trimmed/cut carefully to clear the mounting bolts. The bridge gives the two bolts a purchase to clamp the reed valve and the new adapter plate to the cases. The reed valve's cover plate screws hold the reed valve to the adapter plate.
Anyway, it works quite well. No leaks. Between Saturday and today, Tuesday, she wet sumped a bit. This morning I kicked her over slowly while watching the breather return to oil tank; I made a mess; there was a gusher of oil returning be way of the breather line.
I do not really recommend doing what I have done; the project is fraught with risk of an oil leak that will cause a crash or an engine failure. The CNW method is the way to go.
I'll post some pics later.
