ok, this just happened to me less than a month ago. My car had been running for a few days and I decided to drive it up to a buddies house. at a stopsign it just died and when i cranked it, it stuttered and shook then just didn't fire.
What happened was some dirt that I missed rebuilding the carb got shook loose and is blocking one of passages in the carb. So this means that less fuel is getting to it since its blocked. To fix it temporarily (until you rebuild carb) give it some choke in order to keep the gas-air ratio all nice and dandy.
