1986 suzuki 150SS piston color

[idabassman]

I have owned this motor since new, always ran great. I have a bog above 4ooo rpm. I went through carbs, replaced the primer bulb, checked for air leaks into gas line. It does run better but the bog is still there but not as bad. From wot if I lift a little it momentarily picks up but bogs and maintains that rpm. I pulled all the plugs and looked at the pistons & plugs. The 2 top and 2 bottom pistons and plugs look good, pistons are gray in the middle and wet 3/4 inch all the way around the edges. The center 2 pistons are completely wet with the plugs real wet also. I checked the ohms of the coils and all 6 are the same. I checked the oil injection lines and they are good/clear including the crankcase pressure outlet. I am now going to change plugs. So, my questions are, what should I look for next and why are the 2 pistons different from the other 4?
Thanks,
Idabassman:/