From Port Lines Hobbies

This is not an uncommon problem, and there can be several causes.
a) an overheated motor, requiring rewinding.
b) poor centering of the armature plates within the field plates.
c) too much "play" of the armature from front to back....should have some but not a lot.
d) lack of at least one thrust washer on the front end of the armature, to reduce friction between the armature and the chassis bearing.
e) worn bearing in either the chassis or the brush bracket (both ends of the armature), resulting in too much sideways "play". Replace as necessary....replacement, reproduction bearings are now available.
    f) the face of the commutator may not be flat. It is not unusual for brush-wear to create "cups" or indentations in the surface of the commutator, especially near the slots. When run in the reverse direction, the brushes now tend to get caught, or meet resistance, at those indentations, reducing speed. 

 If (b), (c), (d) and/or (e) do not correct the problem, then a can motor may be a good solution.

If (f) is the problem, you need to carefully "sand"  the face of the armature flat again. The suggested way to do this is to attach a sheet of fine emery paper to a block of wood. Drill a hole in it to accept the short end of the armature shaft. Insert the long end of the armature shaft in the chuck of a drill press, being sure it is perfectly vertical. Now slowly lower the chucked armature down against the emery covered block of wood, smoothing the face of the commutator.

 If (a) is the problem, then use of a can motor replacement may be the solution, especially since it would be cheaper than replacement with original motor components.  If you wish to retain your original motor, then it can be rewound; we provide that service, and all components are machine-wound....not wound by hand.

Another suggestion that has arisen is that the problem can often be reduced or eliminated by adjustment of the smoke gear. First, be sure that the axle gear is properly centered so that it engages the armature worm correctly. If it is off-center, it will not engage the worm in a vertical position, resulting in undesirable torque. Second, be sure the smoke piston gear is firmly mounted....this may require driving the stud which mounts it to the chassis a bit further in. Finally.....and this is the suggested key to the so that the smoke-rod stud is in the aft (rear) position, and the siderod studs on one side are in the forward position. In other words, all the mounting studs are on the drivers and the smoke gear are lined up linearly, again reducing torque.

Can motor installation requires no permanent modifications to your engine.

    Armature thrust washers (three thicknesses: .005, .010, .020)
    Chassis or brush bracket bearings: Inquire
    Can motors:   CLICK HERE
    Rewound armatures and fields: Inquire