syncing only matches the secondary butterfly to the primary one, i can't see how changing jets or moving needles could knock the butterflies out of sync, before there were fancy vacuum synchronizers, a very small drill bit under the throttle plates worked just fine, syncing only keeps the vacuum even, the only way i can see a carb getting out of sync is by the mechanical parts wearing in unevenly, if you see it differently tell me you theory on how butterflies can get out of wack from each other, vibration wearing the tip of the sync screw maybe? but that would be mechanical wearing in, i think?Yes....outg of synch carbs are one of the most common causes of "slow to return to idle" that I've seen, especially if they are way off. One butterfly is open just enough to allow the idle to linger. Missing at higher RPM too....if you are positive everything inside the carb is correct. Synching is a 5 minute process and needs to be done anytime you mess with anything on or in the carbs.
The only way it could suddenly get richer is if the main jets magically got bigger or the PMS (pilot mixture screws) have opened too far.
Go HERE and read...you'll find some very interesting stuff. After that...go HERE to read on how to adjust the PMS correctly....then HERE on how to synch the right way.
Again..I can't see it, hear it or touch it.....it could be a coil...but it's unlikely. The first link I gave you should give you enough material to test that as well.