8142 only runs backwards.

[ADCX Rob]

One of the field wires connected to the reverse unit is shorted to the chassis or cab/shell.

You can confirm this by swapping the brush connections.

[Leverettrailfan]

Everything said so far is worth looking into
I actually have this engine, sort of- it's not mine, it's just in my possetion to be given a service, but I haven't had any rush to get to work on it.

The behavior described with the engine running forwards, but not backwards, makes me also wonder about the reverse unit. Like most 2-4-2 and 4-4-2 Lionel locomotives of its era, It uses a 2 position reverse unit. There's certainly a possibility that the unit needs some attention.
Hair and gunk caught in the gears seems like the most likely culprit of this behavior, all in all it's probably long overdue for a "once over". 
The motor brushes this loco accepts are standard 622-121 slotted motor brushes. These are the most common style of brush used for pre and post war Lionel trains so they're always good to have around. It's definitely possible that changing the brushes could make a difference, but because of the design of the motor, there's not much difference in the tension on the motor brushes between forward and reverse- which is usually what causes the issue you're describing, in diesel locomotives.
Be sure to clean and lubricate the locomotive, not just swap the brushes.
I recommend using Q-tips dipped in 99% rubbing alcohol to clean up old grease and gunk- toothpicks are great for picking at more stubborn material.
I recently stocked myself up with Labelle lubricants, and I have to say I like them a lot more than what I've used in the past. I especially love that they're non-toxic.

-Ellie

[pennytrains]

aboard!

The 8142 came in set number 6-1284 in 1972 and was also available for separate sale.  The set included a number 6-4150 50 watt AC transformer with a  "forward-reverse button".  That button should be comparable to the reverse levers on any AC transformer of the postwar Era.

Since your Loco is equipped with Lionel's Mighty Sound of Steam, the problem is likely with the circuit board in the tender.  Swapping out the board (or even the whole tender) might help.

Cleaning is always a good idea.  Spare parts are available from several vendors.  Try HENNING'S.

[smokey1]

No it's not a postwar engine, you probably buy a used one on ebay cheaper than fixing it. If I remember right it actually has a DC motor in it. I would check to see if there is something binding the motor like lint or other items. Yes it could let it run in reverse okay and not forward as in forward it could be fighting up the lint or whatever around the shaft or something.

Also 8142 was made by Lionel 1971-1972

[Gene 8142]

I am new to toy trains and this forum.  My 8142 engine only runs backwards. In the forward direction, it hums and will go very slow and weak. I thought of replacing the brushes but cannot find a new set of brushes. Also, is this a pre-war engine?    Gene 8142               [e-mail]