A retired carpenter from Kona, Hawaii pleaded guilty yesterday in federal court to making a hoax telephone call to the F-B-I.  Fifty-eight-year-old Timothy David Hershman admitted in court that he called the FBI January 17th and identified another man as someone who was going to hijack a plane headed to Alaska. The man denied the plot and Hershman confessed it was a hoax because the other man put fishguts in his truck.  Sentencing is set for October. He faces up to five years in prison and a 250-thousand-dollar fine.