oil filler cap

The oil breather cap is an early version of crankcase air filter . In other words it should inhale if needed. Any exhale is supposed to go through the pcv valve.
I've seen new pcv valves attached to a dead vacuum line. The vacuum port at carb base fills with carbon deposits first , then the clog can grow on out the interior of the hose. Take that hose off, lay it on the ground and beat on it with a hammer. If you can now dump chunks of crap out of it, replace it. It may crack anyway if its 40 years old.
Now to clean the vacuum port. Twist a drill bit (1/4" I think) into the metal tube then pull it straight out. Wipe the carbon crap from the drills flutes and repeat. You can spary some parts cleaner in it if you want but the more deposits that get sucked in at start up the more likely you fowl a spark plug.
By the way... the mesh material inside the breather cap is fiberglass. Disturb it and it will become tiny whiskers of debrie going into the engine.
Best to buy a new one. I have one that is vented, chrome and marked OIL on my commando engine. I took it apart and replaced the element with the newer crankcase filter material.