The FCR carburetors on my Ducati leaked twice, both times it was because E10 had eaten away the rubber tips (seals) in the float bowl needles. Luckily, motorcycles have petcocks so I noticed it as soon as I opened them up. Haven’t had a problem since I began using ethanol-free gas.
Although if you’ve rebuilt the carbs those needles would have been replaced?? (I have no idea. I’ve never laid hands on anything but SU’s and Dellortos)