From what I've seen, a too-rich mixture will usually generate DARK smoke, akin to the soot a diesel puts out. Grey or bluish smoke is typically due to burning oil; oil smoke in the upper revs and under hard acceleration is generally blow-by getting past worn piston rings, while oil smoke on startup is usually a symptom of leaking valve guide seals. And white smoke, especially if it smells sweet, is coolant getting combusted (Note: if you're just running water and no antifreeze in your cooling system, it won't smell sweet). I've pretty much experienced them all, but thankfully not all on the same car!
Right now, I've pretty much confirmed a bad headgasket on my own D16a6, but it's NOT blowing smoke, and there's no oil in the coolant or water in the oil - the typical signs of a bad headgasket. Mine seems to be pumping air into the cooling system under load, gradually forcing the coolant out (through the overflow bottle) and replacing it with air pumped out through a cylinder. Guess what I get to do tomorrow... (Hint: it involves pulling the head)
Mike
