I remember forum member Bobot posting a topic about this, my legacy k4 is having the same problem ,a small wisp of smoke when activated. I can't remember what the cause or fix was. I hope this doesn't involve an RA #
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It may have just been oil in the coupler. I remember the topic, but I don't recall the resolution.
Well I haven't oiled it myself, if thats something done during assembly thats fine as long as its not an indication of a bigger problem.
You might try compressed air into the coupler to remove anything that might be in there, then see if it keeps happening. If it's oil, it has to run out after a few uses. 
Thanks for your tips John, being a passenger loco I seldom if ever uncouple it, seems to be minimal if at all after some repeated uses, the coupler is not getting hot. I probably wouldn't have ever noticed had the loco not been at eye level in front of another steamer.Ultimately I just don't want anything to deter me from preordering one of those new K Line, I mean Lionel Berks
Hard to imagine anything smoking the coupler before the triac on the R2LC dies.
