Yes I feel like 68 or low 70's is going to be the jet for that carb on a stock setup.
Get an o-ting in there. The cab has to have it. I respect that you had checked for an air leak and did not detect one, but I am still going to advise putting one in. I bought a little o-ring kit from harbor freight for like $5. It comes with all kinds of o-ring, one of which is perfect fit for the sha intake area. They sell a specific formed o-ring for sha, which I have had included in the two 16:16 sha that I have purchased.
Dont be discouraged even if it is something that could have been your fault. Just a learning experience. It is just a moped after all.
Let me tell you how much my immediate neighbors hate me because of all the tuning runs in front of my house.