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Dat73z

240z Tank Vent as Return

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Hi all,

 

After dealing with some high fuel pressure issues I think I've realized my return line from a bypass regulator is too small. Here are the details:

 

1) Stock 73 240z feed into Carter pump, outlet to Mallory/MSD 3psi min FPR

2) Mallory/MSD FPR return to stock return (tiny diameter, restrictive)

3) FPR turned all the way down, solid 5psi fuel pressure at last carb (triple Weber DCOE 40 deadheaded)

 

I'm thinking I should plug the stock return and repurpose one of 3 larger tank vents as the return, but which one?

 

I'm thinking either #1 or #2 in the attached pic. I've also attached a pic of my FPR/pump setup for reference. Check out the tiny return on the bypass side of the reg (bottom). Hoping this can bring me closer to 3-4 psi as testing the reg without the return restriction shows around that pressure range.

 

Thanks!

post-50006-0-76728900-1461246496_thumb.jpg

post-50006-0-67938600-1461246551_thumb.jpg

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I remember fighting similar issues. I finally read the fine print on the regulator I had purchased, and it clearly stated it requires 3/8" feed AND return lines to work correctly. Pretty hard to achieve with a stock tank. It's not just the hose you use, it's the hard lines in the tank that you can't magically increase to 3/8 ID! Mostly I dialed back the pump capacity to realistic levels rather than using huge racing style pumps. Somehow I ran with a stock mechanical pump to feed my triple 44's on my stroker just fine. Now I use a small electric pump in the rear.

 

Remember you NEED a tank vent for filling (larger, to let air escape while filling) and to let air in while the tank during operation, and only the upper two vent lines provide you access to top if the tank for these functions. Later tanks lost that far left side vent in the narrow part of the tank.

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Thanks for the insight, yep 3/8 return stated for Mallory/MSD reg. Also have a smaller feed from the carter pump as well but I am not sure if that makes a difference here. Was running the mechanical pump deadheaded and after dripping fuel everywhere I came to find my mechanical was putting out 14psi! Decided to go electric for feed consistency instead of a deadhead reg off the mechanical. Worst case scenario I could drop the tank and get some larger feed/return lines welded but I'm trying to avoid that at the moment.

 

So it sounds like #2 at the thin portion of the tank is the way to go?

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That actually sounds like a good idea. Was thinking the #2 vent since it would be easy for me to get a new fitting on the reg where I've mounted it and just unplug the existing hose/run a new hose to that location (#2 hose is visible and accessible from under the car).

 

Haven't had time to measure the hose diameters yet but the #2 hose already looks like 2x the ID of the factory return which is smaller than a drinking straw. I'm thinking this should drop me somewhere in the 4psi range for the triples...hopefully lower. Was actually surprised the Carter 4070 pump is putting out so much pressure (~6-8+psi) after reading online that you don't need a reg for it and triples but fortunately my friend convinced me to get a reg anyways.

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Also maybe a return higher in the tank would be better? Looks like the #2 area would be submerged in fuel thus the return would need to fight that additional head? I'll take a look and report what I find. From reading online it sounds like people have repurposed the vents for bypass/return before. Worst case I could get another pump but I'm going to try a couple things before going down that path.

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Im running #1 as my return from my surge tank. Works well. #3 is my vent and all other fittings are capped off except of course for the feed. Evap canister thing has to go. 

Tee your vent into the fuel neck and run the other house outside the car.

 

Put a smoother 90 deg bend on the regulator, may help a bit.

Edited by 240zdan

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If you don't want to smell fuel in your garage all the time, keep the evap canister. Its designed to trap fuel vapors during natural expansion, and pulls that vapor into the engine when running. I get it that they are cosmetically unpleasant but they do serve a good purpose. 

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