Im buying a 240z that is getting rust in the fuel from rust that is in the fuel lines, the old tank has been replaced and now the rust that got built up in the lines is working itself out. How big of an issue is this going to be?

Not an expert here but I had a similar problem in my 76 280Z. I had to clean and coat the tank but prior to this a lot of rust in lines.

I did noticed that the fuel filter in my car showed very dirty/rusty fuel in the inlet side and very clean fuel on the exit side. In other words the filter was getting most if not all of the rust problem before getting to injectors (I realize you have carbs). I replaced my fuel filter a couple times. Also I know some people have added a clear filter in line to see the condition of their fuel.

Jury is still out if I have got all of the rust out of the system but my fuel pressure, added a external gauge, is showing good pressure.

Hopefully others with more experience will respond. Good luck.

Before having the rusty tank cleaned, I bought a length of 3/16" steel cable and tried to run that through the line. Only got it about halfway. Blocked off the end by the tank (partially) and filled the line with alcohol and blocked it completely, let it sit a while, then tried the cable again. Got a little further. Repeated that. Put one end of the line in the drill and spun it while working it back and forth, put more alcohol and repeated that.

Finally got it clear. Hardest part is at the tank end where it turns and hooks to the rubber hose that connects to the tank. Might help to clear that end working from the tank end.

Compressed air might help, didn't have that, though.

Thanks guys all this helps, ill be sure try these things out when i get the car home.