Sounds like a governor system issue that was not connected or calibrated after being built. You did not say anything about idle or start issues so I will forgo the pump timing.
Also, since brand new, it could be that there was a moron in the rebuild center that day and did not set valve lash correctly. Tight valves don't make power.
However, when applying even a small load, you should hear the engine "work for it" and you should see corresponding smoke from the exhaust or some indication it is trying to power up. If nothing, it sounds governor system related whether that is an engine speed sensor, mechanical, etc.