Re: New guy in Cali.
Sounds like you are having plenty of fun, Mac!
The master link gave out because it's china metal.
...just the way things are... some last, some won't
The needle in the slide works in conjunction with the main jet, and slide cutaway profile and controls rpm from abt 1/4 throttle to 3/4 throttle. Then it's all on the main jet, with, of course, a very little influence from the idle mixture adjustment all the way through the scale.
It is most likely you shall need to change the main jet size. I solder them closed and re-drill to smaller, if needed.
A plug change got you 7mph? That's excellent!
Your motorized bicycle will exceed 30mph once you get it tuned better, and of course, broken in well.
Time well spent to review the carb tuning threads!
My goal is cheap, reliable, personal transport, below the tax collector radar. Fun counts for a lot!