i had a problem similar to that with mine and it turned out to be a bad ground cable off of the battery. i can´t guarantee that it´ll be a fix, but it´s not that tough to use an ohm meter on the cable, if you get astronomical resistance, replace it. otherwise, make sure that the choke is opening all the way, that could cause it to run rich, also, make sure that the choke cable is properly adjusted and not frayed or anything. i had to deal with that too.