Oxidation, residue from cigarette smoke or a fire, some other foreign substance on the wheel treads? I had a 249 or 250 Scout (can't remember which) that sparked horribly when it was new and would hesitate to run. Out of frustration I lubricated it thoroughly, and then ran it fast on a wide-radius loop for an hour, every day for about a week! I guess this burnished the wheels, rollers, commutator, etc. The sparking was greatly reduced and it ran reliably with "normal" sparking after that.
EDIT: A few more things you could check... Make sure it's wired per factory specs. (Dual-motored locos like this will run with both armatures wired in series, and both fields in series. I've heard of them being wired this way either deliberately or by accident.) I would also measure the resistance of all six armature coils, and both field coils. Compare them to the default values in Hannon's book. If something is grossly out of spec one motor will probably start and run at a different speed than the other. Finally the brushes... did someone install those high-resistance brushes that are meant for modern Lionels? NOT a substitute for postwar old stock! Good luck in your search, please let us know what you find!
