ORIGINAL: Bob Pastorello
It's far more likely that your specific carb on THAT specific engine has the inlet needle lever inside the carb improperly set. There have been examples where the symptom you report have been absolutely cured by re-adjusting of that lever. If you're not familiar with it, I would contact your seller, and see about trying a "known good" carb to isolate it.
The metering needle lever CAN create exactly the problem you're reporting.
That was my next thought although adding spacers to the standoff would be easy. But if those are the symptoms of improper inlet needle lever adjustment I will check that first.
Thanks for the info.