I am rebuilding a Nikki carb and I am puzzled by one component shown in the attached picture. This nipple seems to be blocked by something other than carb crud. It looks like it’s made of one piece and not hiding some sort of internal part. It appears to be a 1/16 thru hole but so far I have not been able to clear it out. Is it possible that this has something inside that acts as a check? Any info would be greatly appreciated.

This is the nipple in question, it goes in between the two small ones

 

If that is the fitting that goes to the crankcase breather/evap system, it should have a check valve to prevent a backfire from blowing out the crankcase. It probably also checks both ways, so that high vacuum doesn't cause a vacuum leak.
try to blow through it more gently instead of with compressed air.

 

Thanks for the info I will try the low pressure test. What baffles me is this part just seems to be a normal hose barb type fitting. I don’t know how it would have been constructed to have what must be a check ball inside. If I had a a spare I would cut it open see the internal construction.

 

I want to say that is the Sub-zero start assist fitting. I took the hose off of it, and it starts fine in cold weather without it.
I think that stuff can get all cruddy if it builds up, but I don't think it's too important. To my knowledge, there's nothing behind that nipple.

 

You are correct it is the sub zero assist connection point. I realized that after reviewing documentation. No wonder I never had to refill the tank nipple was clogged solid. Not needed now with striped Nikki. Thanks for information.