Weirdly enough, ever since I bought the 750 the Mikuni Mik1 carbs have been leaking on #1. Three new floats and float valves couldn't cure it until I bought a scrap carburettor from a friend and tried using the original float valves with the protective shields installed. Turns out, that the bike must have been parked on the sidestand for a long time and now petrol dissolves the residues and they clog up the float valves. After taking the carbs off a two more times now the float bowls look clean and the carb has stopped leaking. The residue itself was interesting: it looked a lot like some fuel tank liner or something along those lines as it was really flaky and not the usual rust-gunk.
A bit of re-wiring on the sidecar brought both the front and rear-lights back to life.
And this is a special for all those who want to know how to distinguish 64hp heads from the later 74hp heads. (The one shown below is an 850 head though!)
She does sound mighty tasty, mind you (carbs sync'ed and floats adjusted):