Misfires at low speeds

I have a 1997 130 HP Johnson I replaces power head, clean cabs,replaces power pack,had cab adjusted,new VRO pump,new spark plugs, clean oil tank. Now when I shot the gas to her it wil go pushing a 20 ft angler no misfire are anything just runs good like 41 MPH hold on to your hat. When I slow it down it run on two three cyclnders why ? I put a timing light on all plug wires and it is fireing what is the deal. Could it be running to rich at idle or not getting enough air at low rpm. I do have some oil mixed in the gas tank from break in time.

you need to set your idle with the boat in fwd gear and at idle running free. is it a misfire or a sneeze? if sneeze turn idle screw left 1/8 turn at a time, if a cough turn idle screw 1/8 at a time to the right.

Thanks I will try that! Don't quiet know what you mean sneeze. All I know is when I run slow it run on two plugs and no back fireing it does stall sometime. I did turn the two scews the cab to the left 1/8 the lean/rich. Have not been on the water yet. Its got to be something simple becauce it run so good high speed.