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1972 240Z Fuel "Reservoir Tank" Questions

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Hi All,

I'm trying to retain my factory fuel "reservoir tank" (the small metal tank behind the passenger-side rear wheel arch) despite some fairly drastic mods. 


In reference to the attached image:


- I've retained one large ventilation tube (#5) and it's connected to the factory location on the reservoir tank.

- I've eliminated two of the fuel tank ventilation tubes (#6 and #14) due to main fuel tank geometry.

- I've plugged the corresponding inlets on the reservoir tank (for #6 and #14).

- The reservoir tank is connected to the fuel filler hose as shown (and as designed by the factory).


I would like to use hose #13 ("Air supplier tube") as my only fuel tank vent/breather and run it outside the car, well-grommeted through the factory hole just behind the rear wheel arch.


- Is this an acceptable method of venting my tank?

- Is the fuel tank ever supposed to hold positive pressure?  If so, this would make my solution incorrect...

- Should a fuel tank setup ever allow a vacuum to be drawn due to fuel consumption?  If not, my solution solves...

- Is it correct to assume that since #13 is called an "air supplier tube" air must be able to flow:

----> in the blue direction to counter vacuum due to fuel consumption

----> in the red direction to prevent pressurization due to vapor expansion

- Is there a need for a check valve of any kind?


I did attempt to search the forum before posting but failed to find specifics to my questions above.





tank illustration annotated 2.JPG

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Welcome to the forums.


Please take a moment to look in the announcement section. Your post was found posted to the FAQ section and the title contains quotation marks. FAQ section posting does not allow for responses. Quotations, exclamation, foreign characters and such can cause your post to be inaccessible to some users. Both are forum quirks, just take a second to make sure where you are posting and what is in the title and you will be all set!


I must commend you on your well outlined post. It is real clear what you are looking for.


While I am tempted to guess, I will refrain from doing so as it has been a while since I have looked at a stock system. Fuel systems are generally more or less equalized. There is negative draw from the fuel pump and some positive return from the return line, but the vent lines should help equalize pressure, or else you would implode the tank as you drew down fuel according to some people who installed fuel cells incorrectly.

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On my carbed SBC I removed the vapor tank and sealed all of the ports on the tank except the fuel pick tube and one port on the top of the "bulge" part of the tank. I ran that one vent line to the neck of the filler tube so air could escape as the tank was filling up into the "bulge" part of the tank. 


If the tank is completely sealed, a vacuum will develop as fuel is drawn out of the tank. To prevent the vacuum, I drilled a very small hole in the filler cap.


Other consideration: with the vapor tank removed, do not top off the tank when filling up.  On hot days the gas will expand and run out of the cap.  Done this for 20 years with no problems.






Edited by Miles
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It sounds like by drilling that small hole in the cap you've prevented the creation of both vacuum and pressure in the main tank, and it's worked for you.  I think it would follow that if I want to retain this reservoir tank, then using the 1/4" nipple on the top (hose #13) as a breather (with no check valve) would work for me like your solution has for you.  I like the added prevention of spillover that this tank provides.  Thx for the response.


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