I think it's because you haven't updated you mod (the real one).
The latest version uses this name.
You can use this tutorial by
Nick Murdoch
to make it work on any car:
I got Abarth 500 sounds working on mod cars, goes like this:
1) copy .bank from working car, rename it to match car folder.
eg. I copied abarth_500.bank to reliant_robin.bank.
2) add a section to sounds.ini of where the engine is
[ENGINE]
POSITION=front ; or rear I suppose.
3) open content/sfx/GUIDs.txt, find the event section of the car you're copying from, change it to match your own car's folder name.
For example in the test I copied from abarth_500 and changed to reliant_robin:
{0cfb15eb-581b-49e3-a190-6248d359d460} event:/cars/reliant_robin/backfire_ext
{c02355e0-9e60-4a48-b985-7e4b9c20fca1} event:/cars/reliant_robin/backfire_int
{2b7953dc-e66d-4712-800a-34a2b50ea14e} event:/cars/reliant_robin/bodywork
{1d81aee1-d93e-4302-bfac-d102ddebb664} event:/cars/reliant_robin/door
{1d467f20-0702-4ca4-ac3d-e4d938bd6b77} event:/cars/reliant_robin/engine_ext
{b0d135df-9655-4da1-8643-dee1afb4d3ac} event:/cars/reliant_robin/engine_int
{a8a3efde-b054-4bc8-943c-b59926cf9c30} event:/cars/reliant_robin/gear_ext
{a0d612af-ca76-4ab4-9de8-321538b9652b} event:/cars/reliant_robin/gear_grind
{4a80f5ee-44d9-4cfa-b1aa-89d45aaae9dc} event:/cars/reliant_robin/gear_int
{1643425c-c666-4a87-a2b1-c432ce6f2a49} event:/cars/reliant_robin/horn
{582b0a26-cb46-4d47-a1ae-d605b87f80a8} event:/cars/reliant_robin/limiter
{896d78a7-1f58-4409-8d4e-375963e712f1} event:/cars/reliant_robin/skid_ext
{f1d3d869-e641-4ab3-bc72-5cb9928ec118} event:/cars/reliant_robin/skid_int
{fc65915a-abdb-4b06-8232-b5e602e0e2cf} event:/cars/reliant_robin/tractioncontrol_ext
{27ba5b22-3462-43ae-9cdf-62c42f421e6e} event:/cars/reliant_robin/tractioncontrol_int
{794c76ef-eb23-48ba-8c40-db4e83048014} event:/cars/reliant_robin/turbo
{46882e17-772b-4689-9069-c493459ba796} event:/cars/reliant_robin/wheel
{8e8da34b-0156-4064-b6d4-aa9109f68baf} event:/cars/reliant_robin/wind
Save this in your car's sfx folder (content/cars/reliant_robin/sfx) as GUIDs.txt. (note that there are other sections than the "event:/cars/" - I believe they're not necessary when copying from an existing car)
Voila, working Abarth 500 sounds in the Reliant Robin.