Here is a truly simple question that has not been answered: When you are cranking on the engine, what is the system voltage? It's a simple question with an answer that you need to know.
I installed a $12 voltmeter on my Downtown (see pic). But what I now is that if the voltage in the system drops below about 11.1-11.4 v, then you will get symptoms like yours. My bike acts like yours when my voltage reads in the 10's.
Scooter rant in general: Scooter charging systems are fragile. Why they cant put a stupid $5 voltmeter (if it costs me $10, it costs them $1 to make!) on every $5k scooter is beyond me.
Back to the topic: WHILE YOU ARE CRANKING, what is your voltage? Once you have this going, you should also check your running voltage to make sure it is charging the bike.The pic shows my running voltage
Once you are positive that you actually have sufficient power to the whole circuit, and sufficient fuel at startup, THEN start looking at things like a dirty throttle body