1968 Mercury thunderbolt 50hp - stumbles and atalls when warm unless choke is on

[Cynical 1]

purchasing a 68 starcraft boat with the original 50hp mercury thunderbolt engine on it.

runs and starts great until it gets warm, then it loses power, sputters and dies. will start right up, but you need to choke it to keep it running.

faulty fuel pump? dirty carb? other ideas?

thanks.