If you are referring to the screw in the oil tank filler, then my own research suggests that it depends on the condition of the seat and the conical tip of the screw. On my own bike it still sends oil. Suggestions range from a blob of solder in the pipe from the rear of the oil tank to reduce flow through to removing the chain oiler and fitting a blanking plug in the rear of the oil tank.
Vincent rookie here! I can't see how oil could go down the pipe if the screw is tightened down. I can see this thing being difficulat to set up right so I was going to blank mine off and use chain grease.
I've just bought a Iwis mega life chain. Is this a common chain that people use on Comets?
I put IWIS Megalife on from Sprockets Unlimited - fantastic.
The screw has a carburetteur jet taper on it which goes down onoto the female seat. Over time they wear and however tight you screw the thing down it may not seat 100%. You could strip it all and lap it in like a vale seat but I don't think it is worth the fuss. Simply shut it down as far as it will go and fit a blanking plug - helps keep the back end of the bike clean.
I've restricted the diameter of the pipe myself and still get the back of the bike dirty - I think next step is to remove the breather. Better than when I got the bike and there was nothing fitted at all - no pipe and no plug - very dirty!!