Some engines do run at peak with a dark oil color, but generally black oil residue means it's hot/overcompressed. I've had this happen on several occasions where the exhaust residue was very dark when set to peak settings. As long as it's not sounding labored/knocking hard, it shouldn't be a problem. I cured the symptom with a little bigger prop load or running a little rich of peak and just enough compression so it will recover if you throttle back up from idle/midrange.
Black oil residue can also be an indication your ether content of the fuel might be a little low. The ether can "go off" if the can isn't sealed perfectly. I mix my fuel in old acetone cans and I wrap the threads on the can with Teflon tape to keep it well sealed. If you use commercially made fuel, it might not be a bad idea to get a can of John Deere starting fluid to "spruce up" your fuel if it's been sitting awhile and doesn't run as nice as it should.
Last edited by 1QwkSport2.5r; 02-05-2016 at 06:02 AM.