RE: DLE60 Twin
With all due respect, foreign substances don't just show up some time after break-in. It's doubtful that these substances have been in the carburetor the whole time. I'm certainly not picking on you nor the owner of the engine, just trying to help as is everyone else here.
You didn't mention how or if the engine was bolted to a test stand and broke in. Was this done or was the engine broke-in in the air? If the latter is the case, then there is the possibility that there could have been gunk in the carburetor from new but after two cleanings and even soaking it should be clean by now. After the soaking in carb cleaner (not the spray can stuff I hope) the carb body and jets were rinsed and blown out with compressed air, right?
The stopper in the tank is black rubber and not white rubber?