From your post it seems you know what you are doing so I won't suggest you check the air in the tires. I have had the same problem recently with a 1992 90HP as well as a 2000 50 hp PL. You know it takes 3 things for an engine to fire off,--- fuel, air, and ignition. You have at least 2 of these being--- ignition and air. I found out on my engines, and you have the same primer system, that the primer was not priming the engine as it was suppose to. (The 90 would spin all day without firing). After I ensured my fuel bulb was tight, I kept hitting the primer button, probably 30 times and with the start lever up, it started firing. I kept motoring and hitting the primer button and it lit off.
I have never been a fan of OMC's start system but have 2 of them in my yard. I always let my engines run themselves out of gas when I load them after fishing and when I have to start it a few days later I really have to give that primer button a workout.
I am sure you already know about my suggestion. Oh yeah, make sure that primer solenoid at the carb is working properly. I know mine work, but I doubt they are giving the initial amount of fuel necessary for the engine to fire of when I hit the starter.
