Runs badly when fuel tank is filled up

[ajl]

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!

[Royce]

It sounds like the gas tank vent lines might be crimped somewhere. There are several vent lines so I am surprised this would happen, but maybe someone has removed some of the vents in the past. It is common for the vent lines to deteriorate over time and cause gas smell to get into the passenger compartment. If someone plugged a couple of the lines and left only one, and now that one is damaged that could cause your problem. All the vent lines are routed to a vapor tank in the hatch area behind the passenger side interior plastic quarter panel. You could start here but it would be best to get a manual and check out the fuel system, specifically around the gas tank to see a diagram of the fuel tank vent hoses. I remember my friend had this problem with his VW bug and it would actually crush the tank slightly from the vacuum.

Hmm, I suppose you may have problems if your fuel vapor return lines, that run from the carb fuel rail back to the gas tank may be plugged up. This is easy to test by removing the return line at the tank. I cannot remember how high the return line inlet goes up once in the tank so you might want to do this will fuel level is low. Disconnect the return line at the carb fuel rail. Blow compressed air back through the line and see if you get good air pressure out the other end.

In order for gas to get removed from the tank by the fuel pump it needs to have a way to replace the space with air.

Hope it helps!

[260z]

I had similar problem when i changed my carbs. I attached the gasoline recirculating pipe to one of the pipes in the fuel rail that is covered with some heat resistant fabric. Before it did not have recirculating coming of Flattops...New carbs had outlets for recirculating and I assumed that the pipe that was free and most definitely goes to the tank was a recirculating pipe...Apparently it was the breathing pipe from the tank to the carbon can or what ever...This made underpressure to the tank. The car started to run bad and when I opened the tank it said shhh..When it took some air?! Can you hear this sound when opening the tank lid?

I had to pull of the antiheatmaterial to see which line was going where. Then I realized that the recirculation line is just an extension of main fuel rail. It makes an turn in the end..I nmy case the tan breathing line was the one that reaches the backside of engine and is on the top of actual fuel rails.

Just like royce said it has to get the air...

Have you done something to the carbs or took of some antipollution equipment?

[ajl]

Thanks Royce and 260Z. I'll check these things out but not this weekend ... here in Canberra (Australia) we're under alert from bushfires again so my priority (after house & family) is to evacuate the Z if needs be ....

[260z]

Sure hope that they can stop those fires. I cant even imagine how awfull those fires are....Good luck!!