reading Spark plug colors

[rrichar]

What do cylinders 4 and 5 have in common? I am in the process of tunning my carbs after a rebuild. After a decent run this afternoon I took the plugs out to look at their color. Numbers 1-3 and 6 have pretty good brown color. Numbers 4 and 5 look a very rich. After some trial and error I have set EACH carb 3 turns out from closed. So is there some other common thread between numbers 4 and 5? Do these share the same float and float chamber? I have attempted to include some pictures. The colors are not as evident as in person. Numbers 1-3 and 6 look pretty good, 4 and 5 look pretty rich. I can try to turn the idle mixture on these two in to compensate but it just seems odd that these two, right next to each other, are the ones that look very rich when all the others are a good brown. The car is:

1974 911S
RS clone internals
40MM webers
36 venturi
tall A/V
155 main
180 air
60 idles
F3 tubes
MSD 6AL
Bosch WR5DS plugs

With this set up I am still getting some popping from the carbs at 2800 - 3200 rpm. When I went richer than 3 turns (3 1/4) on the idles mixture I definitely started to getting popping from the exhaust. Some charts have suggested with this set up either 55 idles or 175 airs. Does my current outcome suggest the need to change either of those jets?

Thanks for your help.