I have had several hard to start engines. After I went through all of the normal things to do with a hard to start engine, I dropped some solder on the choke butterfly hole. Seemed to work well since they all started fine after that. Seems that that little hole lets to much air to bypass and loose its prime or suction once that fuel is in the carb. I would also check all the plumbing of the tank and make sure the vent line is not obstructed. Some folks put a filter on the vent tube to keep junk from getting sucked into the tank.

Hope this helps.