Why won't my 1975 mercury 65 horse boat motor run at no more than an idle speed when throttle is wide open?

i took my boat out for the first time this season and when i left the dock it ran fine for about a half mile then i stopped and put it in neutral and it stalled. had hard time restarting and limped back to dock at idle speed while throttle was wide open, used last years stabilized gas and put new but slightly different plugs in