Borrow a tank from one of your friends at the field and see how the enging runs on it before throwing out the old. Bad fuel can and will cause engine running issues, but it isn't the only reason for an engine to run bad.

As Jetmech pointed out, if it's stored right, it will last a longtime, I've burnt fuelthat was at leastsix years old and maybe older and it ran like a fresh jug from the LHS. IT was stored in the case the four gallons came in, in a garage. Never frozeand never to hot. You milage mayvary though.

Don