Just to add to what has been said already, the amount you tighten the pivot bolt will determine if it stays where you put the lever. Mine's on a PHBH and will stay at least partially on if I don't pull the lever back.
On the subject of handle bar levers, some question the usefulness of them saying that if the engine is set up right there is nothing to worry about. My argument is that it's peace of mind just to tickle the choke every now and then to check that all is well. On Saturday morning I took a blast up the A11 to visit Byron's Lambretta Rehab emporium. This involves a stretch of about 15 miles of dual carriageway which I covered at between 65 and 70 mph on my Avanti, doing a few choke checks as I went. Arrived at Jarv's and all was good. On the return leg I was not in so much of a hurry so held it down to 60-65mph but after about 5 miles I noticed it seemed a bit 'revvy' so tried the choke which had no effect at all
I slowed down to 55 and tried again and still no effect. Put it on reserve which made no difference but at 45mph it finally had a richening effect. I filled up anyway but continued at around 50mph. When I got home I checked my exhaust flange and could see that it was leaking quite badly but also, thinking about it, it started to rain quite heavily and the temperature dropped several degrees which would have made it run weaker too. If I hadn't had the choke kit I would have continued obliviously and might have holed the piston