I just recently finished a motor swap on my 240z and I have noticed that my car will no longer start if the gas tank is full. I never had this problem before the swap. The old motor was the original L24, the new motor is a 3.1L stroker with SU carbs. I am using an electric fuel pump.
I first noticed that I had a problem the first time I went to the gas station and topped off the tank. After filling up I could not get the car to start so I just assumed that it was vapor lock from the car sitting while filling up, but I no longer think that is the case. I was able to get the car home, but I noticed that if I let up on the throttle the motor would die. I also noticed that the farther I drove the better the car would run. The next time I needed to get gas I filled up the tank and once again the motor would not start. I was able to siphon out some of the gas from the tank and it started right up. It almost seems like if the gas level is higher than the bottom of the fuel fill tube the car will not start. I assume this could be due to the tank not venting properly.
For now I have just been filling the tank up to 1/2 full and I have not had any problems but I would like to actually fix this issue so I dont have to worry about getting stuck at a gas station again having to siphon out gas if I put in too much gas.