TD Exhaust Question

[rdonahue@holli.com (Robert J. Donahue)]

What should the exhaust at idle look like on a properly tuned TD? When I set
the mixture by the "lift-the-pin" method, the exhaust has a little black
in it. By leaning it out a couple of flats, I can make the exhaust completely
colorless. The engine runs great either way. I think I'm in the ballpark, now
should I fine tune towards lean or rich? 

Bob Donahue, Still Stuck in the '50s          
rdonahue@holli.com          
53 MG-TD under DIY restoration
71 MGB still in shop, 9 months and counting