The static pressure in that tiny hose is way less than the suction from the cylinders. It makes no difference at all in AFR. The only difference would be from lack of oil mist.I have a 98 evo head breather motor. To get rid of drippy air filter, I lengthened the outlet hose and ran under frame to the atmosphere.
I was wondering, since originally the hose vented inside air filtered area to carb, it was adding some air to the air/fuel mixture slightly? If you put your hand over the hose when engine is running, you do feel air pushing (and of course oil mist).
I wonder if it affects maybe the air/fuel mixture at idle or so?
I have noticed a little difference on cool damp morning lately.
It's a mikuni hsr42 out of the box for an evo (stage one style headwork and a ev27 cam)
I'm no official mechanic, I'm interested in others opinions