My 914 -74-1.8 runs great but will not restart when hot
Car is tuned well ..new coil. Mixture is too rich when car is hot. Is it the thermostat?

Not sure what the problem is but it's not your thermostat for sure.

Need a bit more information to be able to help:
1. Are you on stock fuel injection system?
2. How do you know mixture is too rich?
3. Are you referring to it not cranking, or not firing when hot?
Again,

Sounds like Vapor lock. Poor cold water on the fuel pump.

It sounds like it’s flooding as if it thinks it’s starting cold. Is it a D jet or L jet system.? If I remember correctly on the DJet there’s a relay that controls the cold start which you can pull to test and see if it will start hot. If so you might want to check the cylinder temp sensor. I think that was the fix for a 2.0 D jet that had the same problem.
Good luck.