Outboard engine hard to start.

SOURCE: Chrysler 70 HP outboard motor dies when pushing throttle

I have found that 99 percent of the time when this happens it is carb related. It is very possible that you picked a a small piece of grit or trash in the high speed jet of one, or both, of your carbs. I would be willing to bet that pulling the carbs and cleaning them will resolve your problem. I am almost positive that will fix your problem, mainly because it sounds like you've already tried everything else. I had this same problem last season, and what had happened was I left the vent open on my fuel tank, we got a shower, and I pulled just a small bit of water into the carb and that was it, she would not do anything over idle until I pulled the carbs and cleaned them. i even put the water muffs on it and let it run in the yard for about an hour thinking that it would burn the water out, but i was wrong. Anyway, clean the carbs and i think you'll be in good shape, Good Luck.

SOURCE: gas flow to outboard

Try replacing the fuel line and bulb itself. They can go bad and it will prime the system but the ball in the bulb will stick causing it to lose pressure.

SOURCE: 6hp Johnson outboard stalls & quits

go to an auto parts store and get a can of gasoline additive called seafoam. Read the label so you know how much to add to your 6 gallon tank. This stuff works wonders on outboard motors.

SOURCE: Yamaha outboard engine. Model f20leha manufactured

have you left the choke open ? this mixes more fuel to the engine upon starting but will stall it if left open too long . check your fuel filter too it may be full of debris.