What jet were you running? Please don't say the one the carb came with......
No, off the shelf comes jetted anywhere near what a moped needs. They are sold as generic replacements and it's expected that you will know the general jetting range. If I remember correctly, sha15's come with a jet in the low 60's? Range. Too lean
Are you running the correct size dellorto intake shim? Did you put an o-ring into the carb mounting clamp throat before mounting the carb?
I'm betting airleak or improper jetting... :( (edited)