I wasn't suggesting that you were not trying to be helpful Anarchy, just suggesting that adding another unknown into the fault-finding process in this case may have lead to further delays and/or frustration.
There are also a couple of specific mounting issues with the MZ installation that differ from yours. One concerns the rubber mounting of the engine, which moves in quite considerably in the vertical direction when running. The carb is rigidly mounted into a substantial cast inlet manifold so that it moves with the engine. The other is the stress on the mounted carb caused by the convoluted rubber air tube. This tube is quite thick wall & quite stiff & is moulded straight. But on installation it is flexed, as the airbox is offset from the carb. The carb is thus under a lot more load than your installation, which is really just suspended between head & airbox.
The BVF carb is a much more substantial lump than the PWK - presumably to carry the mechanical loads the installation imposes on it. Adapting a PWK to fit isn't just a case of knocking up lightweight sleeves or spacers to make up the gaps - the installation will also have to be robust enough to cope mechanically.
The original installation is not infallible - I had a deep seated siezing issue with mine that I eventually traced to a worn socket in the inlet manifold causing an air leak. The only reason I could see for wear here was that the carb had not been adequately tightened at some point in it's life & the stress on the slack joint caused by the air inlet tube coupled with the engine movement in normal operation had worn the socket. The clamps screws could be tightened to stop the carb moving, but the worn socket allowed unwanted extra air to weaken the mixture. I only found this by accident, when one of the clamp screw bosses on the manifold cracked when tightening it enough to stop the carb moving. A NOS inlet manifold cured the siezing issues - strangely the carb spigot was not worn. I guess the diecast carb body must be harder than the sand cast inlet manifold - the two parts having a very different colour & appearance.
Alan - concerns about Molkt quality noted & will be taken into account when (or if) I ever get around to having a play with alternative carbs.
I have noticed that the scooter community have a liking for Dellorto carbs & that there is a Taiwanese Dellorto clone available for around half the Italian original price. No idea if it would be physically interchangeable with the BVF, though.