[Formulajones]

High 13's cruising AFR is perfectly ok. Stoich is only 14:1 with 10% ethanol.

My Z actually cruises in the low to mid 13's and been daily driving it that way for years. Fact is the engine is much happier there as far as drivability goes. If I lean it out light tip in throttle response suffers and low speed power feels less enthusiastic so to speak even though the AFR numbers may look better on paper, the engine doesn't respond well to it. Every engine is a smidge different.

A high gear hill climb you want the AFR to richen, so the fact that your dropping down to low 13's or high 12's is perfectly normal. My guess is the power valve circuit may be opening. That tends to happen more often in high gear and climbing grades where you put a decent load on the engine. Exactly what a PV circuit is supposed to do. Those slightly richer AFR's in that situation will keep the engine safe and has no detrimental affect, other than a little fuel mileage.