They fit Mikuni VM series carbs. I believe one year of the Z1 or maybe even just a certain serial number range came from the factory with those float bowls. For certain, they were also what was supplied when you bought a set of aftermarket racing carbs, the legendary Mikuni vm29 Smooth Bores. They have the convenient benefit of enabling the mainjet to be accessed and swapped without removing the carbs from the bike or the floatbowls from the carbs. Inconveniently, they also lack overflow drains, so if you have float valve problems, gas is either soaking your air filter or going into the oil pan.
