"It just started doing this".... and it's good and cold isn't it?
My 78 Hon PA50 did the same thing on the way to the store with the 90 hp moped guy
That is 2 miles... and it was below freezing... and with the choke on it would run.. and as i went to more throttle the choke would flip off by itself... then it would run about 200 feet and bog and stall... it did that bout 8 times before it warmed up enough to run without choke... it also was still 5mph short of its normal 30mph top speed
If yours will not run without choke after 20 min then it would seem that its dirty inside like you said... restricting the air intake a little would help too.
I think many moped carbs are a little too crude for good running all the time... (mine is.. I think)