Due to the age of these bikes the original plastic fuel inlet on the carb gets brittle and often cracks leaving the serrated brass fitting in the carb, which is an interference fit so would normally be be very difficult to remove.

Mine suffered this fate and initially I figured the carb was now junk, or will cost a bag of cash to get an 'expert' to replace the inlet, however a search on You Tube found this excellent guide on how to remove and replace it which would apply to any carb.

I replaced with a brass one so eliminating the same issue for reoccurring.

Hope it helps;