During my test regimen for the high compression project, I had difficulty achieving good performance at high speed.  My engine ran great in 1st, 2nd & 3rd gear, but towards the top of 4th gear (right about 7000 rpm), my air fuel ratio (AFR) would go max rich (10.0:1), and the engine would simply fall on its face.  Shifting to 5th didn’t help.  It pegged solid & steady at 10.0:1 AFR.

The fact that the instrument was pegged solid & steady at 10.0:1 indicates that the mixture was significantly richer than 10:1.  The AFR typically roams a bit, and under normal circumstances it’s never steady.  The lower limit of the instrument is 10:1.  So, the rock steady reading tells me it had to be richer than 10:1.

A spark plug check also indicated that I had some detonation goin on.  There were tiny silver balls deposited on my ground and center electrodes.  I already knew I had possible issues with detonation since my cranking pressure was very high (about 245 psi).  That’s why I was doing all the testing.

Now I had a double whammy.  I had to put the detonation issue to bed, and I had to figure out why the heck it was going rich at 7K.

The back side of the air filter element had some oil droplets.  That seemed a little unusual, but I still wasn’t alarmed.  I couldn’t recall if I had seen droplets on the rubber seal before.

Although I wasn’t finding anything that alarmed me, the evidence was starting to point at the breather system.  If the breather system was reaching its limit at 7K, it might be spewing enough crankcase vapors and oil droplets to drive the AFR grossly rich.  It would also explain detonation, which can be caused by oil contamination of the fresh charge.  Time to test the theory.

I disconnected the breather hose from the airbox and installed an auxiliary breather filter on to the hose.  Then I plugged the air box nipple.  Did a few laps around the surface streets in my neighborhood and a run on the freeway.  All looked good.

A quick lap on the official closed-course test track was rewarded with a nice 7250 rpm in 4th.  It was still accelerating, but I had to shut down since I was closing in on a Jeep.  The AFR was holding at around 12.5:1.  It clearly had more in 4th , and I imagine 5th too.

But where was that oil going?  You got it, all over the bike.  It wasn’t too bad, but I would hate to see it after an afternoon of this nonsense.

It has a drain bolt on the bottom to periodically drain out the oil, just like you’re supposed to do on the stock air box.  The crankcase vent hose connects to the fitting on the side of the can.  There is a scrubbing pad inserted in the upper part of the can to collect oil droplets and prevent them from being carried out of the breather filter.

Here's what the interior looks like without the scrubbing pad.

The 90° fitting in the side is for PEX piping and you can find it at any decent hardware store.

The filter is glued to a 1-1/2 ABS cap (Permatex Optimum Grey silicone sealant).  The cap is held in place with a small sheet metal screw (#6 x ½”).

Installation is easy and looks pretty good.  Just attach it to the clutch cable with two black zip ties. Sweet!

Seems like you got the AFR problem figured out and explained it all well. Good information there. As with the oil moving out of the breather hose, I don't think anything can fix that as it may come down to too many rpm's for this engine. Unless some catch canister and return system can be made, It's just the way it's gonna be. I run a no-filter hose down to the swing arm  and have no sign of oil there. The bike is geared for highway use and is never near 7000 rpm...I think.  I do wonder if a pcv , ball type check valve at the head cover would help or create too much back pressure.

Hey DBM,
I started a thread about crank breathers a while ago, and got responses like I had two heads. Being very familiar with the BMW (boxer) race bikes and crew, I know how much grief they went through with crank breathing. Those bikes have the pistons up and down at the same time, so there is a pretty radical change in air volume in the bottom end.
Also, having  read more issues of Circle Track magazine than I should admit to, I know that any serious race car is using a vacuum pump in the bottom end to not only reduce crankcase air pressure, but actually run crankcase vacuum.
The new Ducati holy sh-t twin has such an air pump.
In the 70's, Todd Schuster made up a breather can out of a sheet metal paint can (rectangular size) that fit where the starter used to be. He filled it with Chore Boy or some sort of metal scrubby type stuff.
The critical part, however, was the installation of a reed valve from a McCollough chain saw on the crankcase opening. The trick was to not let the air get pulled back in.
Further refinements of the system were done. Udo Gietl (crew chief) came up with the idea of running a steel pipe into the exhaust system at an angle, toward the direction of exhaust flow. This pipe was then connected to the canister. The idea being that the pipe would create vacuum in the catch can.
Which it did.
Another variation was tried by Kenny Augustine (third genius of the Butler and Smith Wrecking Crew). Kenny ran a very small line from the oil catch can to the crankcase top. On piston upstroke, when some negative pressure would happen in the crankcase (remember, it was now a sealed system, with vacuum), the oil from the catch can would be sucked back into the crankcase.
On my bike, I bought one of the OoohBabyOoohBabyOooh Ducati crank breather valves which has two reed valves and a tine return hole.
Just this week I was doing a head scratch session trying to decide how/where to install the reed valve.
So, start by putting some sort of one-way valve on the breather. If you can run it vertically, use a PCV valve from a car motor. If not, use a power brake booster canister one-way valve.
I think you hit a limit when you were fighting crank pressure.
I was actually going to try running a shop vac to the breather line if/when I finally dyno my motor.