Ducati Monster Forum

 I had a Krankvent breather valve on my 240SX Nissan a few years back, and the performance boost was surprising.  When I sold the car I kept the valve and it's now busy improving the performance and mileage of my V6 Tacoma.  I thought I'd try one on my M695, so I took the one off of my '71 Triumph twin, where it was working well.  No change in performance on the Monster.
 Krankvents are most effective on long stoke motors with small crankcase volumes and were developed for use on Harley Davidson race bikes.  These one way valves when installed in the crankcase breather hose create a partial vacuum in the crankcase, so for them to function there can't be any air leaks anywhere that's connected to the crankcase. 
 Connected a vacuum/pressure gauge to the Monster's oil fill hole and it read 0 with a bit of fluctuation as the pistons went up and down.  On my car and truck it would read about 12 in/hg, and the vacuum was enough that you couldn't remove the oil fill cap with the engine running.
 I can't think of a place where air could be entering and preventing a vacuum, unless it's the plastic fitting that connects the breather hose to the engine.  Any thoughts?

How about the hose to the gas tank? Doesn’t it have some type of breather or vacuum hose

There's a reed valve in the crankcase breather intended to keep a bit of vacuum in the crankcase.

Duc motor will have way bigger internal volume than a '71 Triumph, and less pumping as it's a L twin.

Yes, and a 695 being a de-stroked 800, would have even less pumping loss.  But my vac/press gauge reads zero with or without the Krankvent in the line which suggests that the reed valve isn't working either.  Shouldn't I read a bit of vacuum?