Because the squish clearance is too big on the stock head. Once its matched to the piston at approx 1mm, compression is then set as desired. You can have either head modified to the same spec. Good cheap mod, for any bike.
Another thought: Is the Sunoco fuel oxegenated, as around here it comes from a pump right next to the regular stuff? Pump gas has a lot of ethanol now, so if you add all these factors up including cold it could be lean.
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