914 not making power

Thank you;

If there is no air leak, what could be the cause?

The engine has 480 hours of operation.

it may be the 3 way valve on the airbox.  If that is stuck and not opening the air pickup near the inlet of the airbox at full power then the engine will not have the correct air fuel ratio.  Most common is a crack in the black air line.  

Maintenance manual heavy, MMH, has a section on sensors.  You can find all the details there. 

Cheers