It idles up like that with the idle screw backed out all the way. It is not leaks. It is the muffler and boost bottle doing it. It is a 5hp briggs lawnmower muffler with a 1" deep box and no baffles, and just enough back pressure to run it well. I have run it on the middle notch of the carb before and it still did it, but did not run as well. Now it is on the next one down and pulls good and idles ok. I like the high idle anyway for the magneto powered headlight. You can hear it sing when accelerating at all or starting to work under load.
Here is an edited video of the same outing, start at 2:08 for pedal start and fun crackling sounds as it idles down the hill, then pulls away from a stop sign.
moped short clip ride 5min.wmv - YouTube
I'm not even running it wide open up that bridge at the end.
I try to keep around 20-30mph because I don't want tickets. I have a commercial drivers license and it is very valuable to me.