McFarlane carb springs

I know that some owners have fitted these in the UK:

https://www.lasaero.com/products/article/W00WYPOZB

The original Rotax springs are very strong and our throttle friction has to be very tight which I do not like.

Vans RV12's have throttle creep issues. They did go to better throttle from McFarlane and went to lighter springs from them. Replacing the parts on the throttle you have won't help. You would need to use an upgraded and better throttle from them. They do have them as I have installed a few of them. The friction lock is far better. You can use lighter springs and they seem to work. If springs get too light and depending on the throttle mechanism it can make carb sync a PITA because the lighter springs don't do as good a job pulling the throttle levers on the carbs all the way open on both carbs each time you cycle the throttle..

Rotax will not provide you with a formal reference to the issue or confirm their compatibility. Rotax will only provide you with the Service Letter - Non-Approved Modifications or Unapproved Engine Components or Accessories document that you can find here. - https://legacy.rotaxowner.com/si_tb_info/serviceletter/sl-912i-005.pdf

You will need written permission from Tecnam to make any changes to the aircraft including changing the strength of the throttle springs OR the relocation of the throttle springs to be "legal"

The throttle springs are positioned (from the Rotax factory) so that in the event of a throttle cable breakage, it will go to full power (WOT) . Some aircraft manufacturers change the position of this throttle spring to go to full ide in the event of cable breakage and some remove the spring entirely.

It is up to the manufacture of the aircraft to decide how they should have the throttle springs configured and designed so that the throttle works properly and can handle the strength of the throttle spring of the carburetors. 

Unfortunately, this is an issue you need to take up with Tecnam and seek their approval for modification.