I have a Yamaha SZR660 - same engine. Check that the engine breather pipe isn't choked. The engine breather pipe (item 51) allows the crankcase to "breathe" (otherwise the oil seals would seep) and oddly feeds the secondary carb diaphragm operation. If choked in the wire gauze then the effect you have when you blow into the tube won't happen. I'm writing this assuming the MZ uses the same breather system.https://www.yamahamotorcyclespares.co.uk/genuineparts/10717/10/yamaha%20szr660/carburetor?uid=0
The CV or constant velocity carb does exactly as it says on the tin - it allows the airspeed over the fuel pickup (main jet) to run at a constant speed, irrespective of the engine revs. The further down the piston is in the carb bore (venturi), the faster the airspeed - the further up the piston is, the airspeed slows; even though the revs are higher.
The reasoning behind this is that the air/fuel mixture stays more or less pretty constant whereas the other carb requires some mixture compensation device to keep the mixture right. CV carbs have been around for light years on automobiles and up until fuel injection became popular on bikes in the 90s, most Jap 4 cylinder bikes used CV carbs for simplicity.