Return line, fuel pressure regulator or both

[dutchspiderrider]

Hi everyone,

I converted from 1438 single carb to a 1608 double carb 40IDF.
I also switched to an electric fuel pump: cylindrical Facet Gold Flo.

I read some other threads, but stil have some open questions:

1. Since the fuel pump has a max psi > 3.8, I need a return line OR a FPR or do I need both?
2. If a return line is needed, can I reuse the one installed (a thin plastic tube all the way) or install a new one and with what?
3. When I only install an FPR, do I just close the return exits on the carbs and the inlet on the fuel tank?

Thanks for the help!

[bluespider262]

There's like 30 different Gold Flo facet pumps. Which one do you have exactly?

http://www.facet-purolator.com/gold-flo.php

In my experience if its marginally high you will find a proper return setup w/ a metered orifice will actually get you exactly where you need to be.

[sptcoupe]

I recommend a return line, with a metered orifice of .5mm, and a FPR set to 2.0 psi.

The fuel should enter the front carb, pass straight through to the second carb's front inlet, and straight out the back of the second carb's rear outlet to the return line, through the .5mm orifice. Take the psi just in front of the first carb's inlet with this set up.

IDFs are very sensitive to fuel pressure. At 3.8psi you are nearly double the specified psi for the IDF. At that psi you run the risk of the fuel "foaming" in the bowl and a potentially changed float height level as the tabs used to set the float level can bend under too much pressure. You will get a "miss" that is very hard to diagnose because its the same symptom as a bad plug/wire, partially clogged idle jet(s), timing, etc. etc. etc.

A metered orifice will keep the fuel pressure level at all throttle positions from idle to WOT. This is really important to getting max power and excellent tractability.

You can run without a return line, but you greatly increase the risk of vapor lock and heat soak/fuel percolation as the fuel's circulation cools the carb body.

[dutchspiderrider]

Hi guys,

Thanks for the input. For the fuel pump, i looked at the Facet site before, but couldn't identify mine: e11*72/245*95/54*0536*01 and 42731
Since it's a cylindrical the min. PSI is 2.5 at minimum anyway.

I was triggered by the thread: 32/36 DFEV on 78 (http://www.fiatspider.com/f15/viewtopic.php?f=5&t=35675) and the fact my old engine had a return line.

So do I understand correctly the circle of fuel should be:
FUEL TANK >> FUEL LINE >> FPR >> CARB 1 (LEFT) >> CARB 2 (RIGHT >> METERED ORIFICE >> RETURN LINE >> FUEL TANK

Than just two more questions:
1) Which sort of return line (I now have a thin plastic tube)
2) What is a a metered orifice? A link to a vendor with some photo's would help (photo's in general of a Spider with a simular set-up also btw)

I'm kind of liking this journey into the unknown; thanks for the guidence

[bluespider262]

http://rivaracing.com/i-17283646-riva-f ... ictor.html

[bluespider262]

http://rivaracing.com/i-17283646-riva-f ... ictor.html

Or go the cheap route and jb weld one end of a 2" section of 1/4" tube shut and then drill it out accordingly. Start small and gradually open it up more if you find you need a larger PSI drop.

[sptcoupe]

That is the correct routing.

The return line off the rear carb outlet should be standard fuel line. That connects to the plastic line, which you should replace with 3mm (ID) aluminum tubing, back to the fuel tank.

There is no valve/orifice made specifically for this application, but toplessexpat found a nozzle that screws right into the carb outlet and that has a .5mm orifice. Almost like it was made for this application! I believe it is intended for a nitrous kit application. We have used it on our dual carb cars for a couple of years now with zero issues. Topless - do you remember what that piece is made for/where you bought it?

Bluespider's solutions will work fine, or you can also fabricate one from an IDF air corrector jet. Get the smallest one you can find. If you slip it in to the exit nozzle, it will snug up inside and be your meter. Just fill the small hole at the top of the jet with solder, and drill a .5mm hole in the solder. Have done that for years without issues. But the one toplessexpat found is a better solution.

[toplessexpat]

The NOs jet? Autometer 3277 is the part - it's got the right size threaded to fit on the right hand side of the rear IDF and provides a sensible resistance to build pressure against.

Sptcoupe is dead on with his advice here. My IDFs (a pair of 44s on the coupe and a pair of 40s on the wagon) are just perfect. They stay in tune once setup right, and give great power and not unreasonable economy.

[toplessexpat]

Even Amazon sell it!

Auto Meter 3277 Nitrous/Fuel Restrictor https://www.amazon.com/dp/B00062YWX8/re ... Zzb72N0DGH

[dutchspiderrider]

Thanks for the to-the-point. Although for me new territory, I like it! And consequently need your help a bit more.

I had another look at my set-up (btw I bought this complete engine with intake, carbs, outlet, etc. as a complete set), and see the following:

FUEL TANK >> FUEL LINE >> FPR >> CARB 1 (LEFT) >> CARB 2 (RIGHT >> METERED ORIFICE >> RETURN LINE >> FUEL TANK

Is not going to work because my carb to the right has no exit (solid metal):
https://imgur.com/HlkLwLK

On the left one however there seems to be a kind of exit:
https://imgur.com/lQwZTn5

View of both:
https://imgur.com/3MZ8CEL

Can I connect the orifice and return line to that getting:
FUEL TANK >> FUEL LINE >> FPR >> CARB 1 (LEFT) >> CARB 2 (RIGHT) & METERED ORIFICE >> RETURN LINE >> FUEL TANK

Or am I really missing it here.

Searching the net for a self-explaining diagram of a IDF 40 didn’t bring me much up till now, so to me it is still an intriguing black box.

[sptcoupe]

This carb style was commonly used on the late 60's and early 70s on euro cars. I had this set up on one my early 70's cars years ago.

In this case, go from the tank, to the FPR, to a "Y" connector to split the fuel flow from the FPR into two hoses. One hose goes to the front of the front carb (with the fuel return port on it as in your photo). The back of the carb is closed off, and the fuel return line is connected to the fuel return port (the one pointed downward), to the fuel tank.

The other hose goes to the front of the second carb. The rear port is sealed.

So in this case, a separate fuel line feeds each carb, from the FPR. The rear of both carbs is sealed, and the fuel return line comes off the fuel return port on the front carb.

Fiat moved away from this set up because the fuel pressure was inconsistent between the two lines. There was no factory FPR, and the issue they had was constant complaints about a "flat spot" in the transition from the idle jet circuit to the main jet circuit. I had this problem on my coupe. Guys would get around it somewhat by going rich on the idle jets and/or the air corrector jets. Adding a FPR helped some, but when I did that, and put a fuel pressure gauge on each fuel lone downstream of the FPR, the pressure differences were still pretty bad. I even tried a dual outlet FPR to eliminate the "Y" connector, but that was no help.

Later factory cars went to the set up I described above , and the flat spot was nearly gone. Adding a FPR did the trick.

The better set up is the one above, but the original set up will work.

But the idea is always to take the curves and turns out of the fuel delivery plumbing as much as possible - just as you should always try to do the same on the induction intake and exhaust manifold.

Another way to use this carb type is to run a single line from the FPR to the center of the two carbs, and use a 90 degree splitter to run fuel into the front of the front carb and front of the rear carb, and seal off both rear fuel ports. Run the return line form the fuel return port on the first carb (the downward pointing port).

This is will work, but the front carb will lean out when the car is moving at speed, as the fuel to the front carb will have to overcome the rearward centrifugal force caused by the forward motion of the car. This may not be noticeable except that lap times will be slower because the front two cylinders will be too lean. If you are really pushing it, the front carb will lean out enough to cause big misses.

I have helped some guys who came to me with this set up and were having stumbling issues at WOT and highway speeds. Some had fuel pressure gauges on each fuel line, which of course, at idle, showed equal pressure. Of course, they couldn't see the fuel pressure at highway speeds (there is a way to do that), so they dismissed the problem of trying to push fuel forward against centrifugal force. So this is the least desirable way to set up fuel flow, but it does allow you to use the older 13/15 IFD models.

Were I you, if you live in moderate temperatures (not too hot), I would run a line from the FPR to the front of the front carb, straight through to the front of the back carb. Seal off the front carb fuel return port and the rear fuel port of the of the #2 carb, and run it without a return line.

Hope this helps.

[dutchspiderrider]

Hi sptcoupe,

Thanks for the extensive though comprehensive explanation, this is one to keep in my manual and yes it helps! Since I live in Holland, the temperatures here are very moderate on average , so I'l take up your advice on the last set-up (FPR and no return line).

To be continued.