IRZ carburetors were used by Spanish companies in the sixties before they transitioned over to Amals and later Bing carburetors. They seem to be a simple straightforward device but there is not much info on them or how to tune them. The air screw on my Bultaco Sherpa T was opened 8 turns when I took it apart to clean it. Can't be right, I'm used to a spec of 1 1/2 turns for most low speed jets. That info I can't find.
Update: according to the Bultaco Manual 1 1/2 -2 turns ought to be about right.