I'd start by going though the standard maintenance, cam chain adjust, valve clearance, timing just to rule those out.
My understanding is those pods dont ever go well with those carbs, but if it ran just fine for 60 miles possible something else such as a bad condenser, bad advance springs or coils. I always put in new coils condenser and points on the bikes I pick up just to eliminate them as problems in case something like your problem pops up. So far I've been lucky and havnt had your type of problem yet on the past 15 bikes I've got up and running.
Hopefully someone chimes in thats actually had this happen.
I'm with Lefty on checking the timing - if still using points, and if the points haven't been lubricated at the area of contact to the breaker cam recently, the rubbing block on the points can wear a reasonable amount in a short time and cause a narrower gap, retarding timing and making the engine flat-feeling at low rpm. As the revs climb, the advancer brings it closer to where it's happy and it runs better. At least would be a good place to start and as others mentioned, would eliminate one possibility
Aside from what everyone have already stated above I would suggest taking it back out for another ride. When it starts bogging down again, pop the gas cap open and see if the problem goes away. If so then you have a clogged gas cap's vent and it's preventing fuel from getting into the carbs. Easy fix if that's the case.
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