Alf, if the cable comes into the mag high up on the left, the mag is set up anti-clock with left hand end housing, which is ideal. The cable should be tightened, ie pulled at the lever to pull the cable upwards at the mag, to retard as A10gf says. So with the lever fully relaxed (cable extended) you'll be at full advance. I prefer my ignition levers on the left, but that's just a matter of choice and competition for space on the bars. Whichever side, if the lever on the cable is facing towards the outside of the bars, it's forward for advance.
Funnily enough, re Trevinoz' detective work, on both my AMC twins the mags - which are anti-clock like on our BSAs - have the lh cable fitting same as yours, which is what they ought to have from a practical point of view. Space was tight on AMCs under the carb and inlet manifold, and that's maybe why some mags for their twins had the rh end on an anti-clock job. But basically the rh end is better for a clockwise rotator. Plus the fact that there is no guarantee the plate on the mag describes what it is exactly . . . either because anyone can buy a plate and stick it on, or because the mag has been played with (as most have over the years given that so many parts are interchangeable). I doubt any of my mags are exactly what they say they are on the ticket . . . but they all work just fine.
I think you should check that the cam ring is free to move for its full movement - check by taking the end cover off and watching what happens as you pull and push the lever - and adjust to be sure that the cam ring moves the full breadth of its slot against the peg on the inboard side. Worth checking the ignition timing is correct while you're on, see loads of comments in this here forum. Wonderful devices, and when you've poked and played with it, you'll have no troubles that can't be easily solved thanks to the people who support us with spares.
