You can't hear it. You can adjust the pilots by listening but you can't do that with the mains. You can get ballpark by ear and reading plugs but to really dial an engine in you need to know what the A/F actually is at any given time from idle to peak and make adjustments. Only way to do that is with a dyno run or 3 after you get it as close as you can before the runs. I supposed you could install an A/F sensor and try and watch it while you ride to set it up too but too much work and cost, the dyno is much easier.

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