I have seen cen post that they recommend an os#8 or their cen#3 (I think its #3 - whatever comes stock), personally I run an os a5. And to be totally honest I prime to the carby and then a little bit more - like a second worth, just experience gained from blisters over the summer, now I'll usually get it fired with between 1 and 5 pulls.
Are you in cold ambients? If the motor is too cold may it need some help to get warm (hair dryer, heat gun) and it won't help if its too rich. I have read of people putting socks over some of or all of the cooling fins to get the engine up to temp