YAMAHA 150 PRO V OUTBOARD ENGINE

MY YAMAHA PRO V 150 RUNS GOOD BUT AFTER RUNNING A WHILE AND THEN STOPPING WILL NOT TRIM UP. I HAVE SPRAYED THE INJECTORS AND IT WILL TRIM UP. BUT I HAVE 1 SPARK PLUG THAT IS DRY WHEN I CHECK IT. I JUST BOUGHT THIS BOAT AND WAS TOLD IT SAT UP FOR ABOUT 3 YEARS. ANY SUGGESTIONS