Since no one is talking about microcars right now, what Yamaha moped do you have? I fixed my Puch last week, the air intake rubber sleeve had sliped off the carb mouth causing a week mixture and poor performance, refitted it and now goes like a rocket! Well 38mph anyway!!  
  Did the Cold Turkey run on Tuesday, completed the course without wrecking the cabbage fields!
 Did the Cold Turkey run on Tuesday, completed the course without wrecking the cabbage fields!
Yamaha QT 50. Girls moped really, ,looks it mostly!
I hit 27mph once, but thats top really. 
I had the same problem with the rubber sleeve, but before that the wrost problem was when it only ran with choke *and* 2 fingers on the air filter hole (didnt work with filter on), leaving a small gap. Obviously couldnt really ride it! I made my own foam (old one was falling apart, was very thin, but seemed original, and had a couple of holes). My foam was a lot thicker, so wuld ghive it more suction, somore fuel goes past. 
It never seems to go well the first minute it runs, but I have never ran it on the road (apasrt from the road between our big field and our house, since I have no license), maybe needs a good old run (out of the fields) to wear things back in place. When I got it (for free) it sat in a garage for a while, didnt run at all, needed a load of work to get it going.
Our Puch seems to go well, sometimes loses a bit of power, I jsut clean carb, and cl;ean exhaust (gets really dirty, even with correct fuel mixture)
JP