Lionel 682: Burning Smell

Mega Steam makes a smoke fluid that is supposed to clean out the smells of smoke units that have that terrible smell. Maybe try that. Second option would be to open up the smoke unit, clear out the residue and then replace the batting and resistor.

I don't know what kind of train repair guy you went to who wasn't able to isolate the bad smell and correct it for you. Possibly some kid dropped a piece of plastic into the smoke unit and it welded itself to the heater element, now when you run the engine it re-melts and causes the smell. It's not rocket science to rehab the smoke unit if that's the cause of the problem (sniff test while it's running should confirm). If you're not comfortable disassembling your trains then take it to somebody other than the first guy you brought it to.

Disconect the smoke unit wire and see if it still smells when you run it.

If the commutator slots are full of carbon you can get burning of the pheonolic at the ends of the slots. This is usually associated with a loss of power.  If you can look in the slot between the brush plate and the motor frame, sometimes you can see an orange glow on the commutator as it goes around. I think the last time I saw this was on a 671. 

Thanks for all the replies. I did have it apart and when there was nothing out of the ordinary jumping out at me, I brought it into my semi-local, underwhelming train shop. Interesting that Mega Steam makes an "Eliminator" product for foul-smelling smoke units. 

I had a loco a few years back with a nasty odor coming from the smoke unit. I cleaned it out and put in fresh matting and a new element.

Something the original "vintage train repair guy" should have done in the first place...