I have a 1973 240Z with standard 240Z engine, apart from fairly new English SUs. The problem is that when the fuel tank is filled up, there are a variety of running problems (flat spots, stalling, running rich) which disappear as the fuel tank runs down - on a 1/4 of a tank it runs beautifully. Checked out the SUs and they are OK - fuel seems to have trouble reaching the engine from the carbies when full. The engine has a mechanical fuel pump but not an electric one. Any suggestions, anyone? Thanks!