Check the o-ring on "Seat" of the Needle & Seat, sometimes these harden up and may even shrink slightly, and then the Seat doesn't fit/seal firmly in the carb body.
I haven't found it to be common that these o-rings need replacement, but I just cleaned a set of Viper carbs and these are the first ones that the seats just slipped into the carb with no drag, they fit so loosely that if I held the carbs straight up the Seats would have fallen out. It also looked like the o-rings may have been "peeled" or "wiped" a little from when they were originally installed, as there was a small amount of black material stuck to the seat body directly adjacent/below the oring. The black material wasn't all the way around the seat, but approx 1/3 of the way around, and it was similar in consistency to the hardened o-ring.
I imagine that o-ring is metric, I haven't found one the same size yet but will keep looking. I used a slightly smaller diameter, but thicker, standard version that seemed to fit well, at least for now...