[The_AirHawk]

Whether or not it's low on oil should have little bearing on carburetion problems, which is what you're describing.

But don't forget to check (and change!) that oil...........

 

[The_AirHawk]

I just thought it was strange to see the problem in such a specific incident

"I just got an old 1983 XS400R"

I didn't.

"Runs good - daily rider (when I parked it)" are words I've heard often...........

 

[The_AirHawk]

You'll have to ask mscuddy if he knows specifics about this particular bike (and he probably does!), but it might be a carb-synchronization issue, or some spooge in a jet or bleed hole, or simply a quirk of this particular model - it IS an early-'80s moto, after all!