Black smoke after finally catching is likely a build up of fuel, so less likely to be a fuel delivery problem, more likely to be ignition or as I mentioned leaking injector(s). You haven't answered my other basic questions to further help isolate the problem and eliminate possibilities, including too much, too little fuel, or badly fouled plugs. Check the plug gaps while you're at it. When you cleaned up the oil in the plug recesses, did you remove the plug? That might lend credence to the leaky injector possibility. Based on your new info of no longer smoothing out, I'm heavily leaning to the ignition side. I'm guessing two or more cylinders, if not all cylinders, are involved. If it was just one cylinder not initially catching then I'd expect it to start right away, but run extremely roughly until you got normal combustion in that cylinder.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now