Datsun Roadster Forum-311s.org

OK guys, here's a strange one...I'm replacing the 35 year old rotten fuel lines on my 68 2000. As I was unscrewing them from the tank, I noticed that one line was larger in diameter than the other. Larger fittings too. Makes sense. The big one supplies the carbs, the smaller line returns any unused fuel back to the tank, right? Wrong! I did a double take, then a triple take. My return line is larger in diameter than the supply line. What gives? Did someone over the course of the last 35 years somehow mix these up or is it designed that way. Seems backwards to me!

Any thoughts?

Dave

That sounds fishy...

Maybe try putting a little liquid in the tank and blow into each of the two lines. The one that bubbles is the lower and thus the pickup. That's all I can think of since I do not have a later tank to look at...Keep us posted! TR

Good idea, I'll give that a try.

BTW, loved your write up on the DET install. It inspired me to follow suit. I'm heading down to Corvallis to drop off my deposit with Mike Spreadbury on Saturday! I remember seeing a fuel tank with the top cut out in his garage. If nothing else, I'll just hold off on the plumbing until I can look in there and see which line is which. The really bizarre thing is that the lines I removed are dead stock. The larger diameter one runs up the firewall across the tranny tunnel and over to the carbs. The smaller diameter one goes ring into the fuel pump on the side of the block. There's really no way to mix them up!

Dave

I never noticed that arrangement on my '70. Hopefully someone with a later car will confirm your car's configuration!

I would bet that if Michael is modifying a tank, it is an early one...The early ones do not have a return line, but Michael may know.

Good luck with the SR20DET install, Michael's should be getting close to completion so bring a camera, you'll learn a lot from someone that does it right! TR

It's normal - the big line is the return to the tank - SU cars only.

The popular theory around the water cooler this morning is two fold: 1) The larger diameter return allows for some expansion in the fuel due to the heat picked up in its travels through the engine compartment. 2) The fuel up to the carbs is pressurized by the pump. Once it exits the carbs, there's a pressure drop. The larger line ensures the fuel retains a laminar flow and swiftly exits back to the tank.

Sounds reasonable to me...

I think Nissan just liked to confuse people - is it metric or SAE? Let's use both, they'll get frustrated and bring the car back to our fledgling dealers to get fixed. Either that or they happened on some cool machine that make hose with one ID at one and and another on the other end. There are several like that on these cars.

ON BOTH ROAD TOAD'S I'VE OWNED THE LINE THAT IS SUPPOSED TO GO TO THE TANK FROM THE SU CARBS WAS CAPPED OFF. MINE IS CURRENTLY LIKE THAT, AND IT'S CAPPED OFF AT THE TANK. IS THAT A BAD THING? NEVER GAVE IT MUCH THOUGHT UNTIL I READ THIS TOPIC

Tony,

It may lead to pumping too much gas into the float chambers and/or overworking the float needles. This would cause overflow into the intake or onto the exhaust! Also, it could cause fuel to leak from the carbs after you turn the car off. There should be a fuel regulator and a return line to the tank.

TR

You should have it working instead of capped off. That said, it works capped off. The oddball deal on the SU cars rear carb that flows fuel to the rear float chamber and the return line is a primitive pressure regulator. It has a restriction and a spring loaded valve of sorts inside. the idea I think was to avoid fuel heating in the lines while still maintaining adequate fuel pressure at the carbs.

I doubt that capping the line is a big issue. All 1500s, 1600s prior to 1968 and 67.5 2000s came without the return line. (there may be exceptions to that - especially late production 67.5 cars - but that is the way I understand that they were supposed to be.)

Sorry Dave, I didn't mean to change the subject on you. Hope you got what you needed to know. I guess that's why this is called a discussion.......

Change away. I certainly did get the info I needed. I'm off and running now...