Sound like it's not burning or vaporizing the fuel well, you you give it more squirter to supply more light end fuel that will vaporize to get it rich enough, only to find all those heavier compounds loading the plugs up. Gasoline is made up of a variety of components, most usually it starts vaporizing the light ends around 100˚, complete evaporation and the percentages in between depend on the fuel and may run anywhere from around 220˚ to 300˚to completely vaporize. I would guess either insufficient timing, or too cold on the plug, or a bad choice of fuel for the engine. How about some details on the engine and the car, as much as you can, and timing curve as well as the plug type and part #. Here is a couple spec sheets showing the vaporization curves of each to show you.
