Cyl. No. 1 Rich

I just performed an 25 hr oil & oil filter replacement and replaced the spark plugs which had 100 hrs on them. The carb floats were replaced at the previous annual and weighed correctly during this years annual. When inspecting the plugs, the number one cylinder plugs were noticeably darker than any of the other plugs which were uniform in color. This would suggest the cylinder is running rich. How is this possible (#1 is furthest from the carburetor)? Do I need to be concerned? What needs to be adjusted? There are 400 hrs on the engine which generally runs fine. 90% of the time using 100 LL Av gas w/ Decalin additive.