I've grappled with this before, but this makes me think about reconnecting my emissions control again.  Sure I'm protecting the carb, but at the price of losing brain cells inhaling those fumes and messing up the environment?  Doesn't make sense anymore, especially since there is no noticable change in performance.

Early cars- Cooper S, late 998 Cooper, Austin America etc with the PCV valve on the intake manifold (= before charcoal canister and SU's vac port fitted) ran a different oil filler cap. It has a `filter' (more like a strainer) inside, with a 4mm orifice in the bottom to restrict airflow.

I have never replaced mine, I just give it a bath in degreaser every 10 years or so.

Very informative as usual, Norm -

nkerr; the wellspring from which knowledge flows.

reply to jwakil: if you aren't using the system then the cap isn't doing anything other than holding in the oil, so don't worry about it

By the way: the fuel vapor recovery system was the simplest, and lowest mass/cost emission control device, it helped keep an insulated garage from smelling (and the car too), like fuel, and it helps keep water vapor away from E10 gas (with the stupid alcohol which absorbs moisture and causes troubles). The biggest single source of HC emissions from a gasoline vehicle is the vapor it gives off while parked, and the charcoal canister / PCV system reduced that to almost 0, from '68 on. It doesn't need any maintenance, other than about each 10 years to replace the activated charcoal (available at any pet store) and make sure the gas and oil cap gaskets keep a good seal.

Norm

I also vent my crankcase to atmosphere via a small K&N filter, leaving the carb's VC nipple blanked off. 

I periodically clean my oil filler cap; a good shot of brake cleaner through the vent(s) from the inside towards outside and a bit of air blown through followed by a few minutes in the ultrasonic cleaner. 

Rinse, blow-dry and it's been holding up very well, even the gasket.  I don't use an aggressive cleaning solvent in the ultrasonic bath, just the gentle type opticians use to clean eyeglasses.

BTW, I have had the line from the engine to the carburator removed and carburetor plugged for some time.  I have the line that used to be connected to the carb venting to atmosphere through a filter below the car.  Seems the cap would be doing no air flow control in this case.

charcoal canister ?  fuel vapor vent line from gas tank?  Wonder when those features started?  - he said his cap was over 30 yrs old. 

still, I suppose even without those more -modern features, an oil cap metering orifice would limit airflow through the crankcase ventilation route (ie, ventilation air enters via oil cap orifice, mixes with blowby gases, goes out the breathers to the hoses  leading by vacuum to the carb) .

gee, I would think that oil cap orifice would be more likely to get gummed up and close over time,  rather than enlarging.

I am not trying to be argumentative, just learning.

Oldie but a goodie....

When the oil cap is removed, it leans out the air mixture in the intake, and so it is necessary to make sure the cap has a good seal. That is why Rover (and BL before them) said it needed periodic replacement, along with the fuel filler cap (for the same reason).

Norm

I noticed on my oil filler cap on the valve cover there is some writing stating the cap should be replaced every 10yrs or some number of miles (or something to that effect).  I've never had a car where the oil filler cap needed replacing.  I'm sure mine is the original, 30+yrs old.  Do I need to worry about this?