I think I've got it.

Well once I got home and all the domestics out of the way, I pulled the plug to check out if the black had lightened to a brown. It hadn't so I gave the mix screw another increment clockwise, total 1/4 turn from where it had originally been set at, the plug was lightly covered in black powdery soot so I cleaned it up and put it back. When I adjusted the screw, it had a film of fuel/oil/crud on and around it so I cleaned it up and I suspect I may have a small fuel leak, I thoroughly cleaned everywhere I could get without pulling stuff apart, except the air cleaner cover. I rode it to work this morning and just tweaked the idle screw once she was all warm, I am happy now that I have the mix right and the exhaust is definitely the right shade of blue, one more plug check when I get home and that should do it. Now I will keep an eye on a possible fuel leak.

I had noticed this before but thought I'd post a pic anyway.

Pic one. the workshop.

Pic two. some chinese or japanese writing?? not sure why they would be here on an "original" italian engine casing, this may or may not be the real deal I will have to do some research or ask someone.

Pic three. the pic isn't that clear but it looks like someone has taked to the casing in the selector box area with a heavy duty buffer or a grinder to "tidy it up a bit, I guess this part of the bike is quite susceptible to damage and in the 39 years it lived in Vietnam, I'd be surprised if it didn't cop a knock or two.

I will put an order in for some things I'm going to need to get the tank and petcock (gotta love that name) out if I need to, a few gaskets and some spares, just in case.