If it is a choke prob. and you cant get it to open at the right time, you could go back a few years in tech and put in a man choke. Its not a big deal to operate choke funchtion yourself, I kinda like the total control of it.

One more potential possibility, maybe you have a cam lobe wiped out?? This will cause poping through the carb also.

I agree with gordo though, start with the simple things first, usually thats what it is.

Good luck,

Fish