The idle should be about 1100-1200 rpm; lower causes various problems. If you have the idle in that range, adjusting the sync via the vacuum ports should improve it, a lot. Setting the above-idle throttle sync will make it run smoother on the highway, too.
Have you checked the decomp valve adjustments? These usually only cause a problem starting, but can intrude on operations if they are far enough out.
Backfiring on decel is often a lean mixture; the mixture is too lean to fire consistently, and burns in the head pipe, instead. If the throttle sync is off, it will exaggerate the lean condition.
Those vacuum lines may just be the carburetor bowl vents. At some time, they added a vacuum line to one for an engine control, but I don't recall if your bike came before or after that change. In any case, it would be hooked up to a sensor, or be a big vacuum leak.
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'68-7? CB450 and '98 Valkyrie Tourer, too