What happens is when the main metering is lean is as you accelerate from a stop and that will cause a drop in vacuum sufficient to open the powervalve. then as you get going the vacuum comes up and the valve closes that causes the engine to go lean which produces less power and a drop in vacuum which causes the valve to open again and then you get a surge of power and then the vacuum comes up and the valve closes again and so on.
No you need something in the fuel to produce the tan color. Modern gasoline is like white gas they used to sell in hardware stores for Coleman stoves. Also, if the engine burns a little oil it will produce some tan on the spark plugs. The alcohol replaces other components that boosted octane and may have contributed some color on the plugs. Add some Marvel Mystery oil to you gas tank - that should help with giving your plugs some color.
Probably you need to go the other way. Lean it out until you actually get some indication it is running lean.
-jim
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