these were the thing decades ago, but the biggest handicap is you can only use them at idle, more or less. can't rev high and can't load the engine. And judging optimal color is still subjective, everyone sees or understands the colors differently. Nice old school tool for setting idle mixture in the ballpark, but that's all its good for
You would get equally accurate results by listening to rpm changes while turning mixture screw. Or reading real spark plug insulator color after a run. Buy an O2 sensor bung and weld it on the exhaust and use an O2 sensor and a voltmeter, to get a true reading.
