This is known thing when the gear are worn.
It will go smooth in one direction, but feel notchy in the other direction.
This can be in both gearbox and axles.
Usually, bad pot metal gear wears down much faster than quality steel or aluminium gears.
Shimming won't help if the gears are a little worn.
And when you looked at the pinion on the motor, does the teeths look the same on both sides of the pointy end of the teeths?
If the teeths "leans" over to one side, or the teets are thinner one side, it means it's worn and should be replaced.
Also make sure you are using the correct pinion, you have both metric and inch pitch, and also MOD.
Also some wear may be picked up when the drivetrain have resistance, i.e on the ground, table etc, but not holding in the air.
This is a sure sign of either worn gears or low quality production or the quality of material used.
Take off the pinion or remove the motor and push the vehicle back and forth.
Does this make it feel smootly or notchy?
Does it have more resistance in one direction?
If it is, that is surely a sign of worn gears.
I highly recomend to change to hardned gears, from example Axial.
Hope this helps.
Edit: You said turning the gearbox, made the effect reverse, and that is down to the gearbox.
So the gearbox is the problem.
Either pinion, or the gears inside.
Hot Racing among others makes good gears.
I can't say for sure which gears are best as it's about 10 years since I drove a vehicle with a SCX10 transmission, and it had the orginal plastic gears and it held up fine for my use (2s on 55t motor).
These days I have only one SCX10 based rig running a hybrid of HPI and RC4WD transmissions.
And that's the problem with chinesium products, some may work, some may work for a while, but other things are utter crap.
You never know what you get.
But if you switch to Axial or other known brand gears, it should work ok as long as the tolerances in the gearbox are the same as Axial SCX10 which it's based on.
R.I.P Ole (8th Oct 2012)
Last edited by caprinut : 11-23-2019 at 01:47 PM.