BS34 KZ1000 Carbs. Rich

[Foxfix]

I am running a 1981 KZ1000 CSR with BS34 Carbs.
Bike is 100% standard other than a Delkevic 4 into 1 which runs on standard jetting.
Standard airbox with standard air filter.
Carbs are 100% cleaned and refurbished with all standard jets.
Float heights spot on by the manual and fluid heights spot on too.
Bike starts from cold on the button, idles perfectly and runs very well with no issues and pulls like a train.
Bike only covered 11k miles and uses no oil.
Mixtures set with a Colortune Plug to a beautiful bunsen blue (slightly at the richer side rather than lean side)
So...............
Centre cores of all 4 plugs are perfect brown colour, but outer rims of all 4 plugs are dry black.
No matter how I set things (mixtures etc) I cannot get the outer rims of plugs to better colour.
Having worked on most Mikuni Carbs over past 45 years, I have never had this problem.
Is it just the way BS34's are ?

[warren3200gt]

Sounds like you have them set perfectly for ethanol laced fuels.
Dark tan on the earth strap blending to black as it curves to the plug body then black on the plug base with the core insulator the same dark tan again.

[Foxfix]

Thanks Warrren,

That is exactly how my 4 plugs look too.
I only use E5 fuel.

Thought I was going mad, but obviously its just the way these carbs / bikes run.

Appreciate your help as ever.

[zed1015]

Base ring is mixture and ground strap is heat range.
Base ring should have a light turn of soot and ground strap should have colour change on the bend.
If yours are like Warrens pic then you can't get much better.