Positive crankcase ventilation(PCV) needs to be working or Vent hose will send oil vapor to the air filter housing or tube.
All piston engines have "Blow-by", even when new or rebuilt.
When a cylinder fires the hot expanding gases that push the piston down(giving you power) will also push past the piston rings, this is "blow-by", the hot gases vaporize some of the oil on the cylinder walls, this is where the oil vapor comes from, oil squirting out of bearings and valve train is not a "vapor" it settles out of the air fairly quickly.
In "the old days" there was just a tube on the lower block or upper oil pan that pointed down at the ground, blow-by pressure exited out this tube, and with it the oil vapor.
So this tube had a constant drip of oil on to the roads and driveways.
PCV was added to stop this constant dripping, PCV is operated by engine vacuum.
On a carb engine the PCV Valve hose was in the base plate(below throttle plate).
Engine vacuum is highest at idle and blow-by is lowest at idle, so the PCV Valve has a "reverse valve" that passes less air at idle and more air at higher RPMs.
So at high vacuum the valve is sucked in reducing air flow into intake, as vacuum decreases with higher RPM a spring pushes the valve open more so more air is pulled in to the intake.
The Vent hose is there to equalize pressure, older systems just had a vented oil filler cap.
Pretty simple system, if all the parts work.
For the Vent to be sending oil out the PCV system could be clogged up so blow-by pressure is exiting out the Vent.
Or the Vent hose baffle could be bad or missing, this allows splashed oil from valve train to exit out the Vent.
If any are really old, like me, lol, you may remember the "drive carefully after the first rain" saying, it is still partly true today but it was very true 40 years ago.
After a dry spell all the blow-by oil was on the roads, especially at stop lights/signs where cars would sit for a bit, idling.
Oil floats on water.............
When it rained the oil would come off the roads and spread out so when you would come to a stop light/sign it was like driving on ice, wheels locked up and you would just slide, same on some corners.
After a few days of rain most of the oil was carried over to the side of the road, depending on it's slope, but until then there were quite a few rear enders and slides
Still true today but not nearly as bad, cars can still leak oil from oil pans and valve covers.
But before PVC every car and truck on the road left a bit of oil, that's EVERY one.
PCV also does not effect performance, in fact racers play with the pressure, some believe lower pressure in crank case creates less resistance for piston coming down on power stroke, so more power is the result, others think neutral pressure is best so up and down of pistons has neutral effect on one another, results are mixed, but one thing is agreed on, PCV is better than no PCV.
If you have lots of oil vapor then in most cases your rings are loosening up so compression is lower and that does effect performance, the burning of the oil vapor won't effect performance until compression drops too much, but blue smoke out the exhaust is usually from failing Intake Valve Guide seals, not PCV air being burn, valve guide seals leaking is way more common than rings failing.
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