Since I got the bike (about 10 years ago) I have been fighting an oil prolem in the airbox.
Obviously, the oil come from the breather and usually, but not always, the reason is too much blow-by in the crankase.
i rebuilded the top end when I got the bike and cylinders compression and leak-down test confirmed that my issue was not with too much crankase pressure/blow-by.
When new, some Z1 were known to have this "oiling" issue and Kawaski issued a "factory directed modification" back in October of 1975 that consisted of adding a oil return pipe in one of the crankase/breather holes.
This pipe is NLA and I decided to finally try another solution.
My instinct was telling me that the vaccum inside the airbox was at least partly responsible for this problem since it was helping to suck out the oil and air/oil mixture out of the breather.
Instead of connecting the breather to the airbox, i connected it to a oil chain plastic reservoir ( the 75 do not use the chain oiling system) and went for a long ride ( about 100 miles ) with a lot of high rpm's.
Problem finally solved, no oil in the plastic reservoir meaning the issue all along was not a pressure problem but rather a vaccum one.
