Buy

Unusual carburetor flooding issue

Discussion in 'Technical Q&A' started by Peter, Jul 7, 2013.

  1. Peter

    Peter F1 Veteran
    Owner

    Dec 21, 2000
    6,411
    B.C., Canada
    This is one of those situations where I'm completely stumped...

    I've gone through the usual steps to diagnose, but this doesn't make much sense.

    This is a 308 and for those familiar with the set-up, the fuel from the pump is routed through each carb, beginning with the front, driver's side, then the rear driver's side, then the passenger's side rear and finally the front passenger's side. Any excess fuel is routed back to the passenger's side fuel tank through a return line.

    My problem is, with the engine off, I let the fuel pump prime the system and after about 30 seconds, the LAST TWO carbs (the ones on the passenger's side) start to flood. By flooding, I mean that when I look down the carb throats, I see fuel dripping from the bottom edges of the auxillary venturis (the smaller tubes that are in the middle, which deliver the fuel from the main jets/emulsion tube wells). The first two carbs (on the driver's side) are bone dry and not dripping.

    What I've checked so far:

    - Fuel pump pressure is within spec @ 4.2 PSI (book calls for 3.6 to 4.9 PSI).
    - I've bench-tested the pump previously and it delivers the required 100L/hour volume.
    - Return fuel hose is NOT restricted.
    - I've replaced all four float needle valves with brand new ones from Pierce Manifolds and have the float heights set at exactly at spec (50mm from carb top).

    Is it possible I have TWO leaky floats? I can see the tops of the floats through the little vent holes in the carb tops so I think it's unlikely.

    Are the carb tops cracked? The needle valves maybe seating, but is there fuel coming out of somewhere else on the needle valve mounting boss(?).

    I was really stoked to get the car on the road today as the weather is perfect and I've finished the extensive rust-proofing work and other stuff that I've been working on the last several months.
     
  2. To remove this ad click here.

  3. 2NA

    2NA F1 World Champ
    Professional Ferrari Technician Consultant Owner

    Dec 29, 2006
    17,765
    Twin Cities
    Full Name:
    Tim Keseluk
    Try tapping on the carb while the key is on, see if the dripping stops. It's possible that the needles need help seating. Otherwise you need to take the tops off and double check things to see if the floats are floating and the needles open and close freely. A carburetor is like a little toilet, when the bowl is full the flow should stop.
     
  4. Peter

    Peter F1 Veteran
    Owner

    Dec 21, 2000
    6,411
    B.C., Canada
    I tried that, but was hesitant in tapping them very hard... I can try again though.

    Yes, when I called Rob Garven earlier today, when I was explaining the situation, I used the toilet analogy.
     
  5. Steve King

    Steve King F1 Rookie

    Feb 15, 2001
    4,366
    NY
    Sounds like either needle valves are not seating or the float level is to high. I would pull the tops and reset the floats then with the valve seated blow into the inlet to see if it is leaking.
     
  6. Peter

    Peter F1 Veteran
    Owner

    Dec 21, 2000
    6,411
    B.C., Canada
    #5 Peter, Jul 7, 2013
    Last edited by a moderator: Sep 7, 2017
    I re-checked that earlier this afternoon (after I started this thread) and everything is okay.

    Whenever I've set the float height, I always use a dial caliper. I set it at 50mm and check it without the gasket (like it says in the WSM and every manual for a DCNF). See the photo.

    The more I think about this, the more I believe my problem must lie with the last carburetor banjo bolt, which has the return line fitting and a calibrated hole. The WSM calls for it to be 1.5mm, but on my car - since the day I bought it - it was a 1.8mm size. I figured this was incorrect, so I changed it to 1.5mm. I didn't test it back then, but with a smaller hole, it may be causing the fuel to back up in the hose circuit. If it backs up and overpowers the immediate and next carburetor needle valve causing it to overfill (and thus relieving excess pressure), it is probably the culprit.

    No matter what I refer to (online or in print), it's stated not to exceed 3.5 PSI, regardless if it's a single or multiple carburetor set-up. And yet, I have 4.2 PSI right now...
    Image Unavailable, Please Login
     
  7. To remove this ad click here.

  8. FiatRN

    FiatRN Formula Junior

    Nov 21, 2008
    286
    Denver, CO
    Full Name:
    Jonathan Drout
    Peter

    Did you do anything to the carbs before they started leaking, or were they just sitting for a long time while you did other things?

    Did you have the tops off the carbs before the leaks? could the gasket have come unseated or cracked?


    You said you checked:
    - Fuel pump pressure is within spec @ 4.2 PSI (book calls for 3.6 to 4.9 PSI).
    - I've bench-tested the pump previously and it delivers the required 100L/hour volume.
    - Return fuel hose is NOT restricted.
    - I've replaced all four float needle valves with brand new ones from Pierce Manifolds and have the float heights set at exactly at spec (50mm from carb top).

    Did you replace those needle valves 1st, and then it started leaking? Or did the carbs start leaking and then you replaced the needle valves, and they still leak?

    Be methodical - what changed to make the carbs leak? What have you done recently? Is itmore likely to have 2 leaking needles/seats or two cracked carb tops?

    For fun, try swapping carb tops w the leaking/nonleaking cars and see if the problem follows the carb tops...

    Can you test the needles/seats with the carb top off?

    Jonathan
    Denver, CO
     
  9. GT4 Joe

    GT4 Joe Formula Junior

    Oct 19, 2010
    830
    Dana Point, Ca.
    Full Name:
    Joe Williams
    What he said.
    The fuel will take the path of least resistance. The banjo bolt just routes the excess back into the right hand tank.
    With the carb top off, just blow into the inlet (yes, with your mouth, or have your GF do it - <kidding>), and move the float up and down and see if it closes in the up position.
     
  10. Peter

    Peter F1 Veteran
    Owner

    Dec 21, 2000
    6,411
    B.C., Canada
    I thought I should update this thread.

    What I had suspected was what it was.

    Not that long ago, I had modified the return line banjo fitting by reducing it's restriction size, because I had seen in the WSM that it should have had a restriction of 1.5mm (instead of the 1.8mm indicated on the side of the fitting). After confirming with Rob Garven the stamping on his fitting was 1.8mm and a few others too, I took the one back off my carb, drilled it out back to 1.8mm and re-installed it.

    The fuel pressure in the circuit is now below 4 PSI (just at 3.9 exactly) and no more flooding of the float bowls.

    I'm really surprised that the WSM mentions that 1.5mm spec. Thing is, the return line fitting is a common item shared amongst many 308 carb models (and even a few of the 12s too) and regardless of market destination (P/N: 108032) so they should all have a 1.8mm restriction. I guess the 1.8mm size was used after the WSM was printed...
     

Share This Page