1971 240Z, engine won't idle smoothly after fill up...

[Zooter71]

The car: 1971 Datsun 240Z.  Original motor, fresh rebuild (less than 2500 miles.)  Original SU carbs freshly tuned, New spark plugs.  MSD Hall Effect electronic ignition, Pertronics coil.  Electric fans.  Schneider Stage II cam and new springs.  6-2-1 headers with carb heat shields. 4 speed manual transmission.  All smog components deleted.

So, pardon the lengthy intro.  Bought the car Jan of 2023.  Fresh rebuild on the motor.  Always smelled gas in the car, so after researching the forums, it seemed like fuel hoses were the place to start.  Also I saw fuel dripping at the rear of the car. 

So I removed the fuel tank and all hoses and replaced said hoses because I found 52 year old hoses....  While I had the tank out, I checked the tank with a borescope, looks clean. PO must have done some work on the inside of the tank.  This car came with a mechanical fuel pump AND an electric mounted to the mustache bar.  I removed the mechanical and run on the electric full time.

I have changed the electric pump filter and the right angle one under the hood.  I have checked the screens at the carbs.  They're clean.   

I have tuned the carbs and checked them, they are good to go.  The oil in the dampers is the correct oil and at the correct level. 

On a longer trip recently, I found that I had left the filler neck screws in but not tight so I tightened and monitored that for leaks---fixed.

Here's my problem...After I fill the car up completely at the gas pump and go to drive it, when sitting still at a light the engine stumbles...but I can't tell what's causing it.  If I press on the gas pedal to rev the engine, or hold it at a higher RPM (say 1000+), then it seems better, but not 100% right until...after a gallon or less of fuel is burned, then this symptom clears up.

So, only when the gas tank is at its fullest, that's the only time it does it, so I feel it's fuel delivery or return related.

Any ideas?  Of has someone else experienced this phenomenon? 

To be clear, I will be buying a fuel pressure gauge that can sit under the hood permanently and monitor the pressure that way.

TIA!!!

[siteunseen]

I'm not 100% on this but next time your tank is full and it starts acting up, pull over and remove the gas cap. If I remember right it shouldn't have a "whooshing" sound as it trying to pull in air. Your gas cap could be bad, the top vent line could be stopped up?

I have blown into my filler neck, cap removed, and could hear the gas gurgling around. That was my way of testing the hoses after a tank and hose refresh on my '72.

@Mark Maras might have an idea to add. Good luck, it could be a lot worse of a problem. 

[Zed Head]

6 hours ago, Zooter71 said:

So I removed the fuel tank and all hoses and replaced said hoses because I found 52 year old hoses.... 

I'd check the air supplier tube and/or the gas cap.  And the breather tube.  (Edit - actually all of the ventilation tubes.  There seem to be many. Must be important.)  I see that siteunseen has just replied.  Same thought...

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Edited February 26 by Zed Head

[Mark Maras]

Tank venting is a possible cause. Do you know if it's running rich or lean when it's acting up? I always checked mixture problems with the choke first. Ease it on during the problem. If the problem improves or goes away, it is running lean.

[Zooter71]

So, you guys are so helpful!!!!  Thank you so much for the great advice and possible tactics.  I'll definitely try some of that stuff (next fill up) I promise!  This is very helpful!!!  Here is a shot of the engine.  I took off the K&N filters for now, and just have short horns on it.  But man when it's running right, it's a blast to drive!