Like Rob says could be a bad coil or coil wire or moisture or carbon tracking in the cap,just because the carb and filter are new doesn't meen there can't be something floating around in the carb anything could dislodged inside the carb during rebuild or the filter if its paper could have a tear in it and gotten inside I've worked on cars long enough to know NEVER rule out anything,if it can,t possibly happen ,it will"MURPHY" just keep pecking away at it ,let us know what it was Ron 78