Carb Fuel Return: Why?

[EricB]

Hi All,

On our stock SUs after the carbs have been fed there's a hard fuel return line back to the tank.

On the Z I have a fuel pressure regulator set before my triple Mikunis and then the fuel line routes back to the stock fuel return.

My vintage SCCA '73 Civic racecar (ongoing restoration project) has dual Dell'Ortos but the fuel line ends after the second carb - it does not return back to the tank.

Why do some have return lines and others don't?

What would be the harm in having my Z's fuel line end at the last Mikuni rather than return to the tank?

I don't see how the return line affects (if at all) the performance of the carbs.... Can someone enlighten me? Thanks,

-e

[Dan Baldwin]

Fuel is recirculated with the return, keeps it from getting too hot just sitting in the lines. I'm always hearing about triple carbed cars with no return having vapor lock problems, even with heat shields and insulation. I have no such problems using the stock fuel return, though I don't have a heat shield or insulation.

[EricB]

Aaaahhh...... Yes, of course.

Thanks!

Ok how about this as a possibly related question...

The banjo bolts that ship with the Mikunis are of a "T" design, so fuel comes in from the FPR to the first carb, fills that float bowl, then goes on to the second carb, and then the third and then back to the tank...

If I wanted to be sure I had 4psi of fuel at all three carbs shouldn't I then have all 3 carb fuel lines coming straight from the regulator (in parallel) instead of having them routed from the regulator to the first carb then to the second then to the third (in series).

Or is there no pressure drop when they're in series?

-e

[Chino 240Z]

Eric,

We have SUs with no return line and this freaks other Z people out when they see it missing, they say- "well, there is your problem". We are not street driving, so with our setup the fuel doesn't have time to heat up, it's being guzzled at an alarming rate $ (because if it ain't W.O.T., it's parked). Also, as for after a session coming back to the pits to park, we shut off the fuel pump and idle the remaining fuel in the bowls & lines after the regulator. Kinda idle it till it starts to make a change, then kill it. With an improved heat shield, header heat in not an issue any longer. We sometimes run the electric fan for 5 minutes after the car is shut down to circulate the heat out too. Don't really think all these steps are needed, but it don't hurt when its 110 degrees at the track. But never had a problem with heated fuel by doing this.

But I can imagine that city traffic on a hot day can heat up the fuel by the time it gets to and is used by the last triple:paranoid:

craig

* I believe if you had a pressure drop in one they all would see the same drop and it would be read as an overall pressure dop reading on your guage.

[Dan Baldwin]

As long as your fuel pump and lines are up to your hp, there shouldn't be any pressure drop. I used the stock fuel rail with return line on my 3x2 setup. I routed the supply originally meant for the forward SU carb to the front of the forward 3x2 carb, from there to the mid carb, to the aft carb, from there back to the fuel supply originally meant for the rear SU carb. So basically I'm feeding the 3x2 carbs from both ends.

[Chino 240Z]

Now that ounds like a good setup! :classic: