DAMMIT RAY ! (old indian saying)

It was about 80 degrees yesterday, got the 505" fish out for a little drive, seemed to be running better than ever,(close to being broke in). I had just put a 1 step hotter set of plugsin it. Got back after a short trip, shut it off to unlock the garage,
When I started it , it backfired thru the throttle body, w/ a couple of little poop noises in the mufflers. It ran a couple of hundred rpms faster than normal, which is 1150 idle. first thing I thot of was dropped valve, locked timing jumped, but all seemed to be well, I can see thru my glass valley pan. seemed to be running ok, just faster. Next , I thot maybe a ruined part in the throttle body, then maybe plugged fuel filters, or gas level too low and the pick ups uncovered, the fuel gauge was showing about 39 lbs, instead of 43. Two stroke motors will speed up a bit when running out of gas, but never had a car engine do it. Any guesses what happened ????? I`m going to start looking in to it today I guess.-------------DAMMIT RAY!!