My engine was running AFR's in the 12.2 range. My understanding from Marc's writeup and my own tuning experience is this was pretty fat. Marc recommends between 12.9 and 13.1 I believe. I changed my tune to lock in at 12.9 at WOT to 7000 rpm. Coupled with a colder plug and .050 plug gap, the engine runs like a scalded dog and purrs at idle. My experience says Marc is on point. 12.9 to 13.1 AFR is a sweet spot.
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