engine stalls

Hello, I have an Airman  Model PDS 185 S and the engine stalls upon initial air build up, It is not a fuel or air filter issue, It seems like the unloader is not uncoupling the engine from the air pump when proper air pressure is achieved, much like when driving a stick shift car and not pushing the clutch when vehicle stopped.  The engine runs perfect. I would appreciate some input on this. Thank You,  Steve