Hmmm , sometimes different gas will give you different mileage, but thats pretty radical. I've used 1 station for awhile and then switched and lost 4-5 mpg, but not that much. You still have very low miles, weird how it dropped like that but it will still need some time to settle in. The carb
gurus should be wakin' up soon, they should be able to give you some ideas.
Like BIGBEN said...check your choke to make sure its seating right... check out the 'musky choke fix' in tech articles.