You have a Harley Davidson motorcycle with plugs that look rich, but it runs like it's lean and wants more gas! First, if you have any air leaking in at the manifold seals, or the throttle shaft (common on S&S Super B's), you wind up having to jet the low speed rich or run the idle mixture screw way out. Manifold vacuum is high at idle, so any air leak is much worse at low speeds and you get lean pop, as soon as you get 1/4 throttle, the jet is way too rich and foul plugs. Spray something like starter fluid on the manifold seals and on the throttle shaft and see if the engine speeds up. If so, make necessary repairs and go back to original jetting. Next, float level can cause the same problems, but in addition if the carb is more than a few years old, the float can absorb gas and get "heavy" causing the fuel level in the bowl to be too high even though it's mechanicaly set right. If the fuel level is even 1/8" over where it should be the emulsion tube doesn't mix the air/gas in the correct ratio and you can't jet it out. Replace the float with a new one and try again. Last, and easiest to try is this. Unless you live in the desert it should be getting nice and chilly in the winter, and the good high test you run in the summer to keep the engine from hot detonation will now be keeping the engine from reaching operating temp. Switch to regular gas in the winter and check the plugs. Regular gas burns hotter than high test and will cause the combustion chamber to run cleaner. Many folks waste high test in cold weather and on lower compression bikes that don't burn it cleanly. This trick also helps when emmissions time comes around as it can reduce unburned hydrocarbons. Hope this helps!