What is this on the fuel line?

Looks more like some sort of (added) coax antenna connector/mount IMO.

 

As a side note, your fuel accumulator's vent port looks like some one attached a hose and bolt to stop the leaking from the unit..( in the 2nd picture) You should replace it, if I'm seeing correctly,
Gary Bobileff
Bobileff Motorcar Company in San Diego

 

Definitely a coax connector, probably for the state of the art "car phone" from the late '80s or so, having one of those cirlyque antennas on your car was a real status symbol when cell phone time was billed by the minute and cost $0.35 per minute. I remember the first bill I got for my new mobile phone service back then it was a real shocker!
Alden

 

That was the old ITMS system that was used, I could call Europe from the car, and the bill would be less than 1/2 price of my bill if I called from home or the office, also at least a 50 mile transmit range, so no dropped calls, but you're correct, the cell system was just a money maker for every one involved, except the consumer, with terrible performance.
Gary Bobileff
Bobileff Motorcar Company in San Diego

 

Almost certainly, no. A bad Accumulator can only cause a difficult warm restart problem if it is leaking gas back to the tank or onto the ground -- it's not involved in the engine's runability (unless it was leaking so much fuel onto the ground that the fuel supply pressure could not reach spec -- and you'd be on fire ).

 

How "US import legal modified" is your 1983 euro now living in the US? Can you post a picture of the engine bay and the fuel distributor area? Does it have an O2 sensor in the exhaust stream?

 

It does appear as though your car was modified at one time for US importation (I see some added fuel evap system bits and that (now wireless?) crust-o-matic O2 sensor is where a bung for an O2 sensor would be added to a euro), but the injection system modifications look to have been removed so you can refer to Chapter D of the 281/83 Mondial 8/QV WSM for how Ferrari's K-Jet without Lambda system works:

https://www.dropbox.com/scl/fi/twng41f9fpi3ipww7q95v/Mondial8qv_workshop_281-83.pdf?rlkey=5sgpj4yz33jepzrec4x8lcwel&st=uz36lkz0&dl=0

The usual process to confirm/deny the "health" of your system (and fix any found problems along the way) would be:

1. Confirm the regulated fuel supply pressure is OK
2. Confirm the cold WUR control pressure is OK.
3. Confirm the warm WUR control pressure is OK.
4. At warm idle, fully close the air bypass screw and adjust the throttle plate screw to give ~700 RPM warm idle.
5. Then open the air bypass screw to increase the warm idle to 1000 RPM.
6. Adjust mixture (and repeat #4-#6 if anything changes after mixture adjustment)

Good Hunting!