Carb fuel line return restrictor | FerrariChat
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Carb fuel line return restrictor

Discussion in '308/328' started by cbmcdona, Aug 24, 2022.

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  1. cbmcdona

    cbmcdona Karting
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    Caleb McDonald
    #1 cbmcdona, Aug 24, 2022
    Hey everyone, quick question. Is there supposed to be a restrictor in the fuel line between the last carb and the return to the passenger side tank? When I bought my car, someone had completely blocked the return line. I reconnected it, but noticed that the outlet from the last carb is the same diameter as the inlet. I thought there was supposed to be a 1.5mm orifice at the outlet to allow the system to build the 5psi operating pressure, or am I crazy? I noticed that with my return hooked up the car is fine on level ground but seems to struggle for fuel on any hill or off camber if the tank is low.


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  2. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    #2 Steve Magnusson, Aug 24, 2022
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  3. cbmcdona

    cbmcdona Karting
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    Caleb McDonald
    #3 cbmcdona, Aug 24, 2022
    This is good information to have. My exit fitting doesn’t have any orifice at all. The exit diameter is identical to the inlet diameter. So I’m guessing someone replaced this fitting at some point with the wrong one, and is probably why they blocked off the return. Oh the joys of fixing 45 years of other people’s “repairs” lol.


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  4. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    Steve Magnusson
    #4 Steve Magnusson, Aug 24, 2022
    That would be bad ;)
     
  5. cbmcdona

    cbmcdona Karting
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    #5 cbmcdona, Aug 24, 2022
    I would imagine it would almost be better to have the return blocked than have a no restriction return. With it blocked the fuel will get hot and may have issues with vapor lock, but probably better than no pressure at all. I will look for a proper orifice outlet. I have some spare Mikuni 115 main jets that seem like a nice snug fit in my fuel line. Maybe I can try running one of those in the line and see what it does until I can get the proper fitting.


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  6. Rifledriver

    Rifledriver Three Time F1 World Champ

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    #6 Rifledriver, Aug 24, 2022
    Last edited: Aug 24, 2022
    The one in the picture and the only one listed for 308 carb motors by Maranello Classics has a 1.8 mm hole. I have soldered up fittings and drilled a 1.8 hole in them a number of times. The fuel needs to keep moving to prevent vapor lock especially with what we are getting for gas now.

    If all you can get is a banjo bolt, drill it through, get a piece of brake tubing, drill it part way through from the outside a size the brake tube will fit, braze or weld (depending on your skill sets) the tube into the fitting, solder it shut and drill it 1.8. I have done that a few times too. Just do a really good job or it can crack and spray fuel around.

    Or call Pierce manifolds and see if they have any. Ferrari it seems no longer offer it.
     
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  7. dino1971

    dino1971 Karting
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    #7 dino1971, Aug 24, 2022
    What type of fuel pump do you have? Some (like the Hardy pump, which some have fitted) are designed to build up pressure and keep it at a constant level. When installed, the instructions call for closing the fuel return tube. I have this information from one of the engineers that I contacted after I had one fail. It turns out that they are not designed to operate continuously at full speed.
     

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