Rich/Unburnt Fuel

"A fuel pressure gauge for fuel injected engines will come in kinda handy.

A test kit from Harbor Freight is pretty reasonable and has the adapter for the Ford test connection.

With the gauge hooked up to the test connection on the fuel rail, if you turn the key to the Run position, you should hear the pumps run for 2 or 3 seconds and then shut off because the ECM knows the engine isn't running.

During those few seconds, the pressure should come up to 39 psi plus or minus a few pounds, so about 36 to 42 psi. You might have to cycle the key a couple times to get a good representative number.

That 39 ish number will change if the engine is running. At idle speed it will be about 31 plus or minus 3 psi if you get the engine started  When driving it around it'll vary between 31 and 39 depending on engine vacuum.

I'd check your tether switch to be sure it's in place correctly and check fuel pressure as a starting point.

If you don't hear the pumps at all when the key is turned to Run, then it's time to check the 2 relays near the ECM and the 12.5 amp breaker on the back of the engine that supplies the 2 relays. You need 2 good relays for the engine to run, so you cant just swap the relays with each other. Replacing both relays at the same time is the best way to do it."

Another test is to hook up a Fuel pressure gauge.   Turn the key on to pressurize your system, watch the gauge, maybe turn the key off and then back to on a couple times to make sure the system is at full pressure.  Turn the key off and watch the pressure.   It should hold constant for quite a while, even after 30 minutes pressure should be close to the same.   If you watch pressure drop you know you have a pressure leak.   It will either be a leaking injector or a pressure regulator leak usually.  This is the standard for Automotive testing, pretty sure it will work the same for the marine GT40.