I reckon the actual insulator of the plug gets plenty hot enough at high outputs to stay pretty white. On our street bikes we have a pretty hard time leaning on the motor hard enough for long enough to really get things set up properly for max output but it is worth doing and will save your engine. If you use a warm enough heat range plug for your application at high load and wide open throttle it will stay pretty white under those conditions. And you can get a pretty good idea of A/F ratio and timing. But as soon as you reduce the output the temperature at the plug will drop quickly and not be able to produce the same level of information. That is not to say that there is nothing to be gleaned from looking at your plugs at say a medium cruise speed, but the info is in my experience pretty muddy and definitely not the same as at high output. For me, I couldn't care less what the plugs look like at any operating situation except max output. Nothing bad EVER happens tuning wise at less than max output. Get your engine to run well and safely at wide open throttle and you have won most of the battle. Leave the plugs and ignition and main jets alone after that and then fool around with the rest of the carburetion to get it to run pleasantly. If looking at your plugs helps, great, just don't go looking for the same indicators that are so useful at high output.
) and point you here... http://www.hondachopper.com/garage/carb_info/tuning101/carb_tips.html
