It's very difficult to judge mixture by spark plug color since the tree huggers took the lead additives out of gasoline. The old "tan to chocolate brown" color rule of thumb went away at least 30 years ago. About the only way to monitor mixture these days is with an exhaust gas analyzer or an oxygen sensor in the exhaust stream. Plugs will go from white-looking to sooty black (too rich) with virtually no color change from lean to rich along the way. The rule of thumb on headers was once to increase the mixture toward the rich side to compensate for better scavenging, but that was only at full throttle under racing conditions. Other than increasing the underhood temperature, burning spark plug wires, and creating exhaust leaks, I've never been able to measure much difference in actual performance, at least in the RPM ranges most street engines run.
Jerry
