I would not try WD40, as oil is an insulator.
I do not argue with facts, of course, if it did any good
To someone's experience, but generally, a good contact cleaner does just that.
As to your motoring issue:
1. May be the field is weak and shorten somewhere.
You can test its resistance. It should be:
2.8 Ohms for the E2L (60W)
3.2 Ohms For the E3N (40W)
2. The brushes should move smoothly.
Spring tension 16-20 Oz.
3. There may also be something wrong with the brushes.
Are they new? maybe the material is not correct.
There are few kind of materials for brushes.(But probably not the issue here)
If you have an old pair, I would try it...
4. There can also be a problem with few of the armature windings,
So when the brushes are touching either side of it,
There is not always magnetic flux to start motoring,
Unless you give it a "push".
It can be tested with a multimeter, but best to test with a glower.
Reading my post again, I see I wrote "E2L", I meant E3L. of course.
Are you Michael Steinmann, asked here before regarding the sidecar and located in Israel?
If so, if you would like, please send me an e-mail or put your phone No. here,
(I currently have some problems with my cellphone...)
And I may be able help you further.