you need to do a lot of tuning before you can get it to run. DO NOT DROP 12 JET SIZES EVER. worst idea.
you need to run a lot longer, warm up, and ride before your plug is going to tell you anything.
blue smoke, meh. mopeds are smokey :)
your idle or idle circuit could just be off and fucking up your idle. just because teh idle seems to be ok does not mean you have no airleak, i didnt see if that was confirmed with carb cleaner or not...
(of course i am running a 67 jet and it is still rich, but that is in SF on a 2 speed and with totally different diffusers, idle jets, needles and so on than stock.)
screw out your idle screw, lean out the idle circuit. have a choke? use that. then try to get an idle. then search the forum for a tuning guide and tune your idle circuit perfect. and then your idle jet, and then your needle, and then your whatever and then the main jet. elliot posted it recently.