There is a brass protrusion into the float bowl that appears to have no opening - what is it?

I can obviously recognise the pilot and main jets, the other protrusion with an opening I am assuming is the starter jet? But ... there's a fourth that appears to have no opening!

 

That one is probably just a stop for the float bowl pin, I have never came across anyone saying that it needs to be removed for cleaning. Check out the how to section, there is alot of in-depth into carbs.

 

If you're referring to the one in this photo that is not circled, it's the lower portion of where the Main Jet Needle drops down into when the slide is all the way down.

 

I think what I was referring to is just the stop for the float pivot as Mrmanr suggested. Just seemed an odd way to provide a stop!

The other piece I'd like to understand better is the 'non removable' jet in the centre cluster. It has a drilling. On my carbs 2 of them seem to allow a very small bleed through but the other 2 are either blocked or not connected - even sticking a 100 psi on didn't seem to do anything. Can't even be certain the others were flowing as the hose I used wasn't a perfect fit. I had thought it was a feed to the starter jet but not so sure now.

 

Yes, that is the Starter Circuit. I've used a small twistie-tie wire to clear out the opening. If that is plugged, then you're not getting anything for the starter circuit on that carb.

 

Thanks. If a 100 psi from a compressor didn't shift it I'm doubtful I can poke it out with a thin wire but I will give it a go. I guess as it's the starter it doesn't matter if I scratch it. Shame it doesn't screw out ... or does it?