Engine seems to burn oil only after it's warmed up. Rings or valve seals?

I'm getting smoke after the bike is warmed up. Plugs 1&2 have oily residue. 3&4 are dry.

Back story:
1977 KZ650b
Bought it a few day ago. it had been someone's project bobber/rat bike. (It's beyond restoration to original....I bought it mainly for the engine) I won't post a picture because it would make people sad.
Person I bought it from rode it a year ago for about a mile, and it's been parked ever since (indoors, thankfully).
I changed the oil, cleaned the carbs and rewired the bike. it runs now and the starter even works. I do have a vacuum leak at the boots for 3 & 4. Ordering a full set of those today.

I'm wondering if it could be stuck rings, or dried out valve seals.... or if the engine is just worn. Anything else common to these motors that could be causing it? It idles and revs smooth once warmed up, but spits droplets of oil out the 4-into-1 exhaust along with the smoke.

No real common weaknesses on these motors so most likely culprit is just normal engine wear considering the age of the bike and probably worn or stuck rings especially if burning oil when hot.
Worn seals cause oil burning on start- up as oil drains down them when stood overnight and also when accelerating after decelerating as the engine draws oil down the guides on deceleration and then burns it .
Obviously with the bike being laid up a while it may be a combination of both.
If it's running ok otherwise I would put a good few miles on the bike in the faint hope that things improve but in reality you are probably looking at pulling the top end for a refresh.

previous owner claims he didn't get any smoke. I do notice I get MUCH more smoke when twisting the throttle and letting off than I do while it's idling. Also notice I get smoke out of the carbs when I shut the engine off.

I was just looking a little in anticipation of a top end refresh... are piston kits for these really $400-$600?

Just observing the smoke at stand still won't tell you much.
The bike needs to be ridden under load with someone following to differentiate between ring and guide wear if it was just one or the other and are you sure it is oil and not over fueling if the PO claims it didn't smoke previously.
Smoke out of the inlet could mean that the valves are not seating due to excessive carbon build up on the seats or that the valve clearances are too tight.
Yes! quality piston kits are that sort of price.

Valves adjusted?
Steve

Also run a compression test on a cold engine.
First dry and then with some oil squirted into the cylinders.
If the reading goes up that will tell you the rings are not sealing.


.