It should have a constant run unloader. Looks like this.
It will close the valve on the tank side and blow off what the compressor is compressing thru the muffler when it's up to pressure. Usually the throttle is controlled with a 'whip' thru the throttle port also, so that it will idle down when it's up to pressure. If you run it at idle, make sure that it does not under speed the pump. Honda's idle at 1,800 stock, just use a pully calc to determin what you have or need