will winter fuel "ping"?

Just as the title says. I've noticed over the last 2 fuel ups @ to diffrent stations that now out of the blue the ol 273 seems to wanna hesitate a bit & from low engjne speed if you hammer it it pings now! Wtf?! This car has NEVER pinged in the last almost 2 years of being rebuilt! Timing hasn't changed same stock afb I'm open for ideas. idles fine no miss that i can tell at all. not using oil or coolant i really wanna blame the fuel but @ two diffrent stations having sub par 91?!? Thanks people.