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Carburetor, fuel pumps and fuel pressure problems

Discussion in '206/246' started by DinoLasse, Mar 6, 2019.

  1. DinoLasse

    DinoLasse Formula Junior
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    May 26, 2009
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    Lars
    There seemed to be no end to the problems I was having with the idle mixture adjustment on both new and old carburetors in my M-series.

    The symptoms were: Idle mixture needed to be repeatedly adjusted. Idle rpm would suddenly drop, almost stalling. Engine seemed to run rich, even though mixture had just been adjusted correctly.
    Looking down the carburetor barrels I could see fuel trickling down above the throttle plates in one of the DCNF carbs. Repeated adjustments of the float levels did not solve it. Changing the needle valves did not solve it. I assumed (incorrectly) that the stock carburetors had not been correctly rebuilt and installed the new 44 IDF carbs. Initially everything seemed OK but soon the same symptoms appeared - except 10 times worse! Fuel was now trickling down from the accelerator pump jets on two of the three carbs during idle. Continued to trickle down for 10 or 20 seconds after the engine was shut down, depositing half an inch of gasoline on top of the throttle plates! Had to siphon the gasoline out from the barrels with a hose.

    This was getting serious, so I decided to temporarily go back to stock DCNF carbs and solve the underlying problems first.

    Long story short: It was all caused by excessive fuel pressure. Found to be 5 psi going into the carbs. Too much. Workshop manual says regulator is preset to 0.25 bar (3.63 psi) Turning out the adjustment screw all the way on the FISPA regulator/filter only brought it down to 4.5 psi. (The original FISPA FRB 11 only has an approx. 1 psi adjustment range) Leaving the pumps running for longer times, pressure slowly crept up to 6 psi. Way too much.

    1. Found the FISPA FRB 11 fuel pressure regulator needing rebuild. Membrane old and stiff, probably a small pinhole leak somewhere.
    2. Also found the fuel pumps installed were Facet Red Tops. Not ideal for the Dino, producing up to 7 or 8 psi. Should have been Silver Tops, with max 4.5 to 5.5 psi.

    Those two factors combined caused all the grief. The high fuel pressure simply overpowered the carburetor needle valves and gradually pushed up the fuel levels in the float chambers. The longer I had the engine idling, the richer the mixture became. No wonder it was driving me crazy.


    Temporary fix was to install a Malpassi fuel regulator at the inlet to the carburetors. They are adjustable over a wide range and work very well. I set it to a very low 2.9 psi (0.2 bar) to be on the safe side. The idle now became steady. No more flooding issues.
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    Permanent fix: Out with the old, bye-bye to the Facet Red Tops. Will not be missed.
    Lovely original FISPA FRB 11 will be restored and saved for the future.
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    In with the new: New Bendix look-alike pumps with correct pressure rating. New Malpassi Filter King pressure regulator/filter. Not exactly like the FISPA but close enough for now.
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    At first I planned to just put the pumps in parallel to duplicate the original look and function. But I like the idea of having one pump as a backup to the other, and to be able to run either one pump or both.
    To make that possible, I had to install a one-way valve in the output line of each pump.
    It is only a minor deviation from the original setup, and easily reversible.

    Searching this forum, I found that Adrian (alhbln) had already done a similar setup, except for a different reason (vapor lock) and with Pierburg pumps. Details here: https://www.ferrarichat.com/forum/threads/new-fuel-pump-setup.372894/


    Later I plan to install two hidden toggle switches controlling the pumps under the driver's seat, for example.



    Here is the setup installed in the car.
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    I set the pressure set to a conservative 3.0 psi (0.21 bar). Now it always stays at that value, engine running or not.

    Recent warm and dry weather last week enabled me to take the car for a three hour drive. It drove very well and the idle was always steady and healthy sounding. So far, so good.

    In conclusion, the issue of fuel pressure is rarely discussed here, but my experience tells me that it is quite critical in the Dino, and is becoming more critical as components are aging and original parts are replaced with aftermarket items. Currently available needle valves for example, seem to vary quite a bit in their ability to hold off fuel pressure. Even the factory preset value of 0.25 bar (3.63 psi) seems borderline too high now. Experienced carburetor people seem to agree on a range of 3.0 to 3.5 psi (0.21 to 0.24 bar) as ideal for the Weber carburetors, and no more than that.

    Fuel pressure is easy to check and well worth keeping an eye on.
     
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  2. pshoejberg

    pshoejberg Formula 3
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    Dec 22, 2007
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    Peter H
    NIce write up and valuable information Lars! I am still running on the original fuel pumps to my knowledge and have often considered if I should change them out. Good to be informed what to look out for with respect to fuel pressure and how critical the correct range is. Staying original and stock is often the best on these cars but it is becoming increasingly difficult as NOS supplies run out.

    Best Regards

    Peter
     
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  3. dgt

    dgt Formula 3
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    Jan 14, 2011
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    Agreed, useful info Lars.
    I had the other problem with the Facet pumps from Dennis McCann, not enough pressure and one of the pumps was bad out of the box and the second pump was pushing fuel back through it.
    I ended up buying the repro Bendix pumps you have, they look right and work well.
    I measured the pressure (although finding an accurate guage can be tricky) and found these are set internally to 2.9 psi which is a little lower than factory spec but seem to work well on my car.
    Good to have a second opinion because I was concerned the pressure was a bit low.
     
  4. DinoLasse

    DinoLasse Formula Junior
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    Andrew, 2.9 psi sounds just about perfect to me. The only reason to go higher would be if you run into fuel starvation at extended full throttle runs. I don't think we are even close to that limit, but I am hoping to find out when I take my car to the rolling road dyno. If there are any such signs at full power, I plan to increase the pressure until the starvation goes away - and then add some safety margin. I doubt that it will be needed, though.

    Did you measure your 2.9 psi after the regulator? If so, you should be able to increase the pressure by turning in the adjustment screw a bit - if you wanted to. My pumps measured 5 psi directly at the outputs before the regulator, with either one or two pumps running. If you have the same pumps, it seems to me that they should produce similar values.
     
  5. Nuvolari

    Nuvolari F1 Veteran
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    Sep 3, 2002
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    Excellent information and a reminder of the importance of proper fuel pressure in a carburetted engine. Thank you for sharing I am certain this will be a valuable reference to more than one owner.
     
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  6. dgt

    dgt Formula 3
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    I never ran into fuel starvation and this was measured after the regulator, the regulator is fully out so it's not regulating the pump but just filtering.
     
  7. DinoLasse

    DinoLasse Formula Junior
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    Good to hear. Didn't think so either. Those pumps have a lot of volume capacity, so fuel starvation should not be an issue, even when running with just one pump. With two pumps there should be all kinds of excess capacity.

    With regards to the regulator, when you say fully out, you don´t mean the adjustment screw, do you? Because if it is fully out, it is set for maximum reduction. To be fully out of the circuit and just filtering, the screw should be all the way in. A bit of Italian logic, I suppose.
    I hope I am not insulting you by stating something obvious here. If so, please forgive me, Andrew.
    Best, Lars
     
  8. dgt

    dgt Formula 3
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    Ha, no I mean’t screwed in...
    I forgot which way to turn it but the diaphram should be pressed down into the body of the regulator via the adjusrment screw on the top to increase the fuel pressure, correct?
     
  9. DinoLasse

    DinoLasse Formula Junior
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    Yes, absolutely right. You screw it in to increase the fuel pressure. Or you could say, the pressure reduction gets less and less until, fully screwed in, you pass on the full pressure from the pumps.

    If you are already at that point, and only get 2.9 psi, it probably means that you managed to find a version of the Bendix look-alike pumps that are set to such a low value that you don't need a regulator. Makes a lot of sense, some dealer/distributor could probably do it. Apparently the pressure can be changed by simply replacing the spring inside. But when I was looking for the Bendix type pumps, I was only able to find one version of them, set 4.5 to 5.5 psi. And that is pretty much what the original Bendix no 476087 put out, so I was happy with that.

    If you have such low pressure pumps, it should not be a problem that I can see. In fact, your car should work just fine even if the regulator goes bad, as it did in my car!
    2.9 psi (0.20 bar) is regarded as perfectly adequate even for a sports car engine. For a standard engine, 0.17 bar (2.47psi) is considered adequate.
     
  10. dgt

    dgt Formula 3
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    Lars, everything you said is correct, the pumps I bought came from Australia. A porsche guy modifies them to make “more accurate” reproductions. It’s been a while but I recall that he sets the pressure.
    They are also replated in gold CAD or zinc which was how my original pumps came (some early Dinos were plated in gold, some silver) and with early type labels.
     
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