Cougar SV smooth running gearbox

[Q_man]

Hi All,
I started my Cougar build last night and it is such a great car to build, everything went together so smoothly. But then I stopped at the Gear box.
Are there any tips on getting the gear box to run smoothly?
I have currently just installed the two inner gears in the gearbox and mounted it onto the car.

I have read about running it in and have tried to run it in on a dremel for a few min on low and it did help a bit but still sticks on one side. Another tip was to file small amounts from the plastic gear to free up the sticking side.

Just wondering should I just keep running it in or pull it down and start with the file, any other tips?