This is a new engine?
Its easy when installing the injector pump to not install the throttle arm in the fork like part of the govenor.
If you bought it new, they may have stuffed up at the factory. If you don't install the arm in the fork, you essentially get full throttle and the stop lever will do nothing as its not connected to the pump. Hard to explain but makes total sense when you see it.
If you take the stop lever plate off, you *may* be able to see it. The injector pump has like a rack setup. Its a bar with teeth cut into it, this slides left to right. The further to one side the more fuel you get, the other lower and goes down to zero that shuts the machine down. If the pin on the rack is not in the correct part of the govenor it will race and can damage the engine from over revving.
If that is the problem and you pull the pump to get it in the rite place ensure you use sealant around the pump when you re-install it, if not you will get oil seeping out around it.
http://users.tpg.com.au/tazza_/kub/Untitled2.jpg
It shows the pin i'm talking about, item 10.