DCNF....yes, more DCNF...

Discussion in 'Workshop Forum' started by pafiat, Jan 14, 2020.

  1. pafiat

    pafiat True Classic

    Location:
    australia
    since we are having an open season on DCNFs..My 1500 runs dual DCNFs, yes they are a PITA, but, the noise makes me forgive everything. I live up a reasonably steep driveway, the instant the nose points up the hill at quite slow speed the car stutters, gulps and falters. This is resolved by sticking the foot in it and rocketting to the carport. Is something wrong with the carbs or is this a DCNF "feature". I am running a manifold with correct angle of the dangle for the X1/9 and float levels are set as per my Haynes Weber handbook at 50mm.
     
  2. Dr.Jeff

    Dr.Jeff True Classic

    Location:
    Sin City
    This is just a very general comment in response to what you described.
    That can be typical of any oversized carb arrangement (e.g. all dual Webers, etc). They often have a flat spot off idle/low RPM, especially under load (going up hill). This is one advantage of a single carb arrangement, unless of course that single carb is also way too big. The trade off for having all that carb is improved top end, WOT, high RPM performance.

    That being said, you might be able to reduce this with some carb tuning. Webers have a huge array of replaceable components that affect various aspects of their performance. It's a rather involved topic but there are some great books on it.
     
  3. carl

    carl True Classic

    Location:
    Virginia
    Ahh...another rap on dual Webers! The notorious off idle flat spot on dual IDFs for spiders can easily be tuned to eliminate and I see no reason why this situation can't be tuned out. Dual Webers came on many cars right from the factory and I would assume they were set up for all normal street situations. You probably have the wrong size idle jets in those carbs.
     
  4. Sounds like you may have an issue when going from idle through the transition jets to the mains. I found that this can be a strong function of the emulsion tubes and float levels, as well as some effect from the mains. What emulsion tubes and jets are you running?
     
  5. pafiat

    pafiat True Classic

    Location:
    australia
    idle:50
    main:120
    air:220

    the problem really only manifests itself at small throttle and pointing uphill. Its fine on the flat and going full tilt.
     
  6. You might want to check the float levels, and also see if they are moving freely. That, coupled with where the emulsion tubes have holes will affect the transition region. What emulsion tubes are you running?

    The jets you listed look in the same range as what I'm running. Currently, I've got 130 mains, 55 idle, 220 air with F36 emulsion tubes. I think F24s are also popular for this application.
     
    Last edited: Jan 15, 2020
  7. pafiat

    pafiat True Classic

    Location:
    australia
    Thanks, I'll check emulsion tubes. My motor is fairly stock so jet sizes look about right. Do you know what float height you are running?
     
  8. Ulix

    Ulix True Classic

    Location:
    Stuttgart, Germany
    Do you have the carbs mounted the right way around?
    I have seen them mounted backwards, with the fuel bowl towards the rear of the car. This could potentially cause the problem described.
     
  9. 50mm (recommended default). I did have to run it higher with some other tubes.
     
  10. pafiat

    pafiat True Classic

    Location:
    australia
    good thinking Ulix but mounted correctly
     
  11. carl

    carl True Classic

    Location:
    Virginia
    If it runs fine other times but does this going up a steep drive, then the float bowl should be full of gas just before you run up the hill. Definitely check float level. Does this appear to affect both carbs? I don't have an answer just making sure we know all the facts.
     
  12. Which manifold? Is it a 128, an X1/9 or one of the dual purpose manifolds.

    The 128 has the engine canted forward more than the X, so the manifold is angled for the carbs to sit level. Put that manifold on the X and they are canted back too far. Add to that the steep driveway and the emulsion tubes are likely starving for fuel. Add that to the regular off idle stumble and its a real problem.
     
  13. Ulix

    Ulix True Classic

    Location:
    Stuttgart, Germany
    That doesn't seem to be the problem either...
     
  14. carl

    carl True Classic

    Location:
    Virginia
    Then again, exactly how steep is this driveway?
     
  15. artz1731

    artz1731 True Classic

    Location:
    Denver PA
    My 42DCNF has been sitting on top of an Alquati 128 manifold since I bought the car 10 years ago. It never misses a beat.
    I never understood the theory that the Carburetor needs to be perfectly level. If that were the case You could never park on a incline....or drive up hill? Not much flat land here in the Northeast.
     
    autox19 likes this.
  16. carl

    carl True Classic

    Location:
    Virginia
    I'm sure in theory a carb should sit vertical but even the dual IDFs that came from the factory on Euro 124 spiders had a tilt to them.
     
  17. pafiat

    pafiat True Classic

    Location:
    australia
    driveway is about 50m long with quite a tilt to it, not sure of the height gain. havn't had time to hook out the emulsion tubes, later next week. thanks everyone for your ideas.
     
  18. carl

    carl True Classic

    Location:
    Virginia
    Just for fun, what happens if you back up the driveway...trying to figure out if carb orientation plays into this.
     
  19. artz1731

    artz1731 True Classic

    Location:
    Denver PA
    Uhh...I wouldn’t do that. The reverse gears aren’t very stout. Backing a distance uphill is just asking for trouble.
     
  20. pafiat

    pafiat True Classic

    Location:
    australia
    ha, I like your thinking Carl! As noted though, not sure the gearbox would be too happy. One thing I did notice is that the cold start jets are at the back of the carbs. They are not used but is there anything in there that might not be sealed properly and might flood the motor when more immersed in fuel than normal
     

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