There's no screen or diaphragm or anything on the inside of the engine behind the nipple. That hose just feeds openly into the crankcase and gets it's vacuum signal from there. I'd take the hose off, clean it out or replace it, and shoot some compressed air into that nipple to completely blow it out. If that doesn't work, your fuel pump may have taken on some oil so you may have to clean it out too.
Here's the nipple on one of my motors, I'm assuming this is where your fuel pump vacuum hose hooks up (since you said you have a late '80s cart):
Here's the view from inside the motor:
This little hole is what's on the other side of that nipple:
