You ran w/ "bad gas" doesn't mean much... Any damage is likely done.
If low then just add more fresh fuel.
if full then empty most then fill w/ new.
Use a bottle of injector cleaner or
"dry gas" can help w/ a full tank of fresh fuel. Run close to empty then repeat. (Fiero tank is small and using both in same fill-up can be bad.)
Replace fuel filter after running awhile. Fiero filters take allow of crap before plugging. Crap can come out of the tank after filling once or a few times.
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
The Ogre's Fiero Cave