A few years ago i bought an s-gauge engine, from the 30’s, 40’s.The smoke fluid went into the tender. For awhile it ran good, then got very slow. Took it to get serviced. It was cleaned and some new parts were added. THE guy who cleaned it said it ran really good and had some speed. I got it back, ran it a little bit and started running slow again. Could it be the track? Anybody got any ideas?
I can’t help you with your slow running; but I think your locomotive dates from after the war, when American Flyer began using S gauge. They introduced the smoke-in-tender scheme in 1946.
I think your problem is a worn out armature, if it also runs hot when it runs slow, then it is a dead giveaway. If not, its still probably the cause. I would recommend getting a new armature from an s gauge parts dealer or ebay. Good luck.