I agree with Silversurfers comments. I think that plug is on the verge of being fouled. It should not look black on the electrode or ground pin. It should be a tan color and dry. Your plug looks black and a little shiny--perhaps just a bit wet or oily.

The mix ratio shouldn't make your plug look like that. I run 40:1 and my plugs look tan and dry after many gallons of running. I've seen plugs puled from engines running as heavy as 24:1 and they look tan--not black as yours is.

You've been running rich on the top end. Put the new plug in and adjust it again.

Check your plug gap on the new one. Too much plug gap will make it hard for the plug to "jump" to ground and make a good spark. Too little gap will just give it a weak spark. When you get the gap in the sweet spot--it's a nice thick blue spark. I run my plugs around .028" ---.032" gap