No issues with the igniter box - methinks that carbs could be gummed up, try some carb cleaner stuff or pull the carbs and clean with carb cleaner and compressed air. There is a possibility that the rubber spigots(between carbs & manifold) have deteriorated and cracked causing a leanness but unlikely, worth a check though. Hows the air filter, that might contribute if it's real dirty - does'nt sound like ignition, though the low tension connectors to the coils can become corroded - a simple oxidation scrape and a little electrical grease can solve that issue - thats a tank off jobbie - the coils are under there.
Does she do the same on the prime setting at the petcock(remember to set back to normal/reserve if that is not the issue). Good luck!
