Fire problem ... maybe

With what you have described it sounds like a fuel problem, that when the fuel pump is run at a higher speed than idle that the pump pressure rises to the point that the carb and or the needle and seat can't shut it off and this is where your getting the black and blue smoke from. Too much gas and removing the oil from the cylinder walls creating the blue smoke from oil. I would look at the carb and fuel filters first. The way that I see it is that if the ignition system was failing that it wouldn't change from running well to running poorly, also too much gas will give a misfire as the plugs get fouled. Remove the plugs and see if this isn't your problem. Just some thoughts.