Had the same issue on my port 470 engine so I rebuilt the 2bbl carb. and it works good now. There is a small ball at the bottom of accellerator pump bowl the holds the fuel in the chamber and the last person in the carb had left it out so I replaced it.After reinstalling the carb the engine then started right up. I started up both engines the other day and they both required pumping the throttle three or for times and cranking for about thirty or forty seconds , had not started them for about three weeks or so. Choke on port side is sticking closed a bit so ran rich until I adjusted it and the engine warmed up. The starboard engine has always started and ran fine. If I remember correctly you have to put the throttle all the way forward to set the choke.