My buddy has a 2004 merc O.B 150 and he never primes it doesn't even have a bulb so he says I find that hard to believe, I always prime mine but I'm having a issue she is not getting hard, or firm I will hold it horizontally next time but I don't remember ever doing that before ,.
Great question. I wish I had a good answer. 1980 Mercury 200HP. Runs great, idles good, but rarely starts without starter fluid. I squeeze the bulb until hard. I can see the fuel in the line with a new inline clear Moeller gas filter. New Moeller fuel/water separator, new Attwood bulb. Not sure if I am not providing enough fuel to the engine or flooding it. After I run it and let it sit for a couple of hours (while trolling), it starts (usually).