If you were spinning 5600 before and you're only turning 4600 now (no changes except for the decarb) then you're firing on 5 of your 6 cylinders.
Pull all the plugs and inspect the white ceramic part . . . all of them should be paperbag brown in color. One will be white. That's the one that isn't firing.
Replace that one spark plug and/or make sure the coil and plug caps are on tight. Take her for another spin.
Oh yeah . . . You probably have to gap your plugs on the OMC motor . . . so make sure that your in spec with the gaps.
See the carbon on the ends and the paperbag brown color on the tip. That's good . . . The one that comes out looking brand spanking new (from a nice petroleum high pressure clean). That's the culprit. Fiddle with the wires and the plug cap for that plug and you'll be back up at 5600.