First of all, I think your machine is not made for what you are trying to do....13mmm is about an half inch or so and this is knee lift....your actuall sewing lift is near 1/4 inch.....maybe good for light upholstery or heavy garment.
You need a larger machine, like an Adler, Cobra or Cowboy,,,,your machine needs to do at least 138 or 207 on top (thread size...) and do 92 or 138 on bottom for 2 layers of 5/6 latigo!!
Do some research on this forum, trade your machine back in or sell it, or keep it for very light leather....then call Bob Kovar, or Steve or other Sewing machine sellers here on this forum....they will guide you in the right dirrection!!
And try to learn as much as you can about leather sewing machines...!!
Guess how I know all this....haha...been there done that.
Greetings
Jimbob