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308 GT4 Weber DCNF questions

Discussion in '308/328' started by Stefan Elshout, Feb 18, 2020.

  1. Stefan Elshout

    Stefan Elshout Karting

    Dec 1, 2011
    146
    Holland
    Dear people,
    I'm currently struggeling with a 308 GT4 and i hope anyone over here can help me with some of their knowledge.
    The car had a pretty bad backfire problem. First i went over the distributors (dual distributors with normal points setup), rebuild them, set them up on my distributor testbench so i'm sure they are working perfectly. New spark plugs, HT-leads. Coils and extensioners also checked on testbench and all is working fine. This made quite an improvement but it did not sort the backfire problem.
    Then i checked the obvious like fuel pressure etc. etc, noticed it was pulling false air at the base of the carburettors and the intake manifold so decided to remove and rebuild the carburettors/intake manifold. Took it all apart, ultrasonic cleaned them, replaced gaskets, needle valves, set float height, repaired the linkages, synchronized them and adjusted the mixture screws with a colortune spark plug.
    All fine and good, engine fires up at the first revolution, idles perfectly, no backfire at idle, all stable.

    It also drives quite good. Changed the idle jets (from size 45 to 50) to reduce the lean spot during progression phase and that improved as well.
    Strange enough with very little load on the highway it keeps backfiring. It must be somewhere in the progression phase because it is only with little load between 100 and 125km/h. It really feels like a lean backfire.
    Than i decided to put 55 idles in and slightly bigger main jets but the backfire did not went away.

    I read that i am not the only one here, but is there anyone out here that can point me into the right direction with my problem?

    One strange thing i noticed, when the car is idling all is fine. When i let it run around 2000RPM on idle, fuel starts dripping out of the acceleration pump nozzles. Than i thought, hey there is a 'check valve' inside the pump delivery valve (the screw that bolts down the pump nozzle) that might not seal. But it is not spring loaded so it is not supposed to seal if from fuel coming thru with low pressure but instead from air coming in.....
    Is there anyone that noticed this phenominom as well?

    anyway, long story but i hope there is someone out here that can point me into the right direction. It would be very much appreciated.
     
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  2. absostone

    absostone F1 Rookie
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    I would like to know the answer to your question as well.
     
  3. Stephen B.

    Stephen B. Karting
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    Nov 3, 2018
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    Check your coils for proper output. Had a similar problem with my GT4. Found one coil faulty. Put two new Bosch red labels in. No more problem. Good luck with whatever is wrong.
     
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  4. Stephen B.

    Stephen B. Karting
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    Further note: Try to determine which bank of cylinders is backfiring, forward or aft. Swap the coils and see if the problem moves. If it does then your coil is bad. Again good luck
     
  5. Stefan Elshout

    Stefan Elshout Karting

    Dec 1, 2011
    146
    Holland
    Thanks for the replies.
    Checked both coils and they both function well. Had the complete system (distributors, wires, coils, extensioners) mounted on my distributor testmachine and nothing strange to see there. I could try swapping coils but I'm very allergic to replacing parts that are not proven to be broken.
    If it would be ignition related i would expect the backfire not to be so specific to a certain load.
     
  6. TonyL

    TonyL F1 Rookie
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    You clearly understand your car very well and tend to agree it could be carb related & could be in the progression phase, but can you describe the fault in more detail. I had a similar problem on my 246 and kept getting snapping and hesitation at about 40MPH. I finally cured it with a process of Fine tuning of the carbs.

    You say backfire but does it come from the exhaust or carb throats?
    When you set up the air flow rates what did you settle on at 900rpm?
    Is anything breaking down under load
    Air bypass screw settings?

    Tony
    PS dont go to large on the idle jets without altering the AC jets, 45 to 555 is a big jump. In my opinion the jetting from Weber for these cars are pretty much spot on given the calibre of fuels on the market. I stick with shell optimax 99RON and standard jets.
     
  7. Martin308GTB

    Martin308GTB F1 Rookie
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    What do the experts think regarding false air through the throttle bearings? Or did you replace those with sealed bearings?
    I also had -slight- popping and hesitation issues while driving at around 2500 rpms under steady load.. Disappeared after my carb rebuild including sealed bearings.

    Best from Germany
    Martin
     
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  8. Stephen B.

    Stephen B. Karting
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    Nov 3, 2018
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    Stefan, sounds like you are doing everything right. When I had my problem it was at idle so it was easier to troubleshoot. I know you have checked and adjusted your points and all good but in the old days on our muscle cars it wasn't uncommon to get "point float" at high RPM's with a misfire. The points drift a bit when the dizzy is really spinning. We would set the points a little closer than normal. I've never heard of a Ferrari V8 have this problem though. I decided to take the points situation out of the question and installed a electronic ignition in my GT4. Not saying you should do that since you can't really determine whether it's a fuel or elec. My guess it's probably fuel at this point.
     
  9. Albert-LP

    Albert-LP F1 Veteran
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    I have not the skills to rebuild the carbs by myself, so I went to a carburator specialist to have the 308 (GTB) carbs redone. After that the car still had some misfire and several other problems. Then I changed carburator specialist and I had them redone another time. The cost was three times the first attempt, but the car then worked perfectly: no misfire, of course, never.

    There are many things that must be perfect in a carb, or it won't work well. Changing some parts and cleaning them is not enough: the good specialist changes every seal, boost pump or bearing and even bores the carb diffuser, puts new bearings and so on. Then it works.

    And don't think that "old cars don't go well with today gasoline" like the first carburator specialist told me... the second one showed me how the work had to be done and the car still works like (or even better) when it leaved the factory almost 40 years ago

    ciao
     
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  10. Stefan Elshout

    Stefan Elshout Karting

    Dec 1, 2011
    146
    Holland
    Thanks a lot for all the answers.
    @TonyL : - Backfire comes out of the throats. I'm familiar with some pops and bangs in the exhaust with different cars but that is less concerning than real backfires in the intake to me.
    - Flow rate on my flow gauge was around 9ml/min at idle. Don't know if that makes sense but i did not pay to much attention to the actual flow but more to get them even with the others.
    - What you mean by 'breaking down under load'?
    - Air bypass screws, 6 of them fully closed and i needed to open 2 screws between 1 or 2 turns.
    - I went back to the 50 idle jets. 55 did not make it any better so i changed them back to 50, which was a bit of an improvement over the 45 but only in terms of driveability and less in relation to the backfire issue.

    @Martin308GTB : I did not replace the bearings with sealed units. Are the sealed bearings a factory revision of the open bearing? I did ultrasonic clean the bearings and refilled them with thick grease, so i would be surprised if it pulls false air thru them....at least for now that is.

    @Stephen B. : I did check for points float. I put both distributors on my distributor testmachine and they functioned flawless up until 9000RPM (which is the maximum RPM my machine can run at) without any signs of float. I think this specific points/spring/cam assembly works quite good to run high RPM's

    @Albert-LP : Yes i'm sure there are some much more capable guys that know these Webers much better than i do, unfortunately i don't know them. I can only bring them back to as good as original condition, balance them perfectly, adjust the idle mixture correctly, do some basic work with jets and troubleshoot most obvious problems. Given the fact that most cars run without problems this way it should theoretically not be necessary to anything drastically like boosting pumps, drilling extra progression holes. At least not to sort my problem i would assume. But ok, praxis and theory..... you never know.


    No one ever noticed fuel dripping out of the acceleration pump nozzles with stable but increased idle RPM? It almost looks and feels like at that point there is a resonant frequency of back pressure in the intake manifold. I feel like that back pressure gets somehow into the accelerator pump circuit and pushes very small drops of fuel out of the acceleration pump nozzles. It 'drips' with somewhat equal RPM as the engine is running with at that moment. That is the only weird thing i noticed that i could not place and might point towards my problem....???
     
  11. Martin308GTB

    Martin308GTB F1 Rookie
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    Jan 22, 2003
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    Because you filled the bearings with thick grease I no more think, that they are the culprit. The grease will again dissolve over time, but freshly applied it should seal the bearings enough.
    AFAIK the sealed bearings are not a factory revision, but an upgrade based on common sense.

    Mine do that too. A little bit. I always considered it irrelevant but actually one can see mixture getting richer over prolonged idle periods.
    I think, it also has something to do with expansion of the fuel in the float chambers and acceleration pump, while the carbs get heated by the engine and slow fuel consumption. It even drips after shutting down the hot engine. That's the cause of repeated black smoke after restart of a hot engine.
    Is the amount of dripping uniform on all accelerator jets or is it different?
    After 24 years I got accustomed to some carb special behaviour :)

    Something completely different regarding the behaviour of your engine. Did you check timing? Not ignition. I mean vale timing. I don't know, how the 308 engines behave, but I have seen and experienced an Alfa Nord engine with valve timing a few degrees off and similar symptoms. Not enough for valves and pistons to collide, but enough for running poor.

    Best from Germany
    Martin
     
  12. Stephen B.

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    Stefan, As an old aircraft mechanic I always tried to live by the rule "don't over think the problem, keep it simple stupid". Much easier said than done though. Have you considered a possible sticking intake valve? Your symptoms tend to lean that way since you seem to have ruled out so many other possibilities.
     
  13. TonyL

    TonyL F1 Rookie
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    #13 TonyL, Feb 21, 2020
    Last edited: Feb 21, 2020
    Thanks Stefan.

    My synchronizer is rated in kg/hr and i normally set mine around 4 to 4.5 kgh/hr (ml/min seems a rather low / odd calibration to use) If you just concentrate on purely air flow balance without keeping an eye on rpm (aim for 850rpm) then you have a tendency to open the throttle plate/s to far and thus bring in the 1st progression port. Under low operating conditions this will then advance the accelerator pump circuit /s and possible (will) flood the intakes with fuel (more than needed ) ie fuel dripping from the accelerator jets. This will give you snapping out of the barrels. I preset my throttle plates using a feeler gauge between the diffuser bore and butterfly. A .003 gap will get you in the right ballpark.

    For me, this is your problem!

    A loose wire may allow the ignition to work at tickover but when being driven hard it may break down due to the extra demand placed on it - i call that breaking down under load. Lok for any evidence of weak wiring joints, connectors etc. But this sound s like carb issues to me.

    2 turns is a bit much, was that on the same carb?

    Tony
     
  14. MiuraP400

    MiuraP400 Formula Junior

    Feb 3, 2008
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    I have seen retarded ignition timing cause both the accelerator jet fuel dribble and the delayed cut in of the main jets that is causing your backfire at 100 mph. Hemi heads like a lot of advance. Typically you need 42 degrees total advance on a hemi head engine. I my experience this will not only to the two issues you are having but it will also unlock a lot more performance from the engine.

    I am not sure how your distributors are set up. Do you have dual points? If so are they phased properly? That can also cause retarded timing on some cylinders.
     
  15. 2dinos

    2dinos Formula 3

    Jan 13, 2007
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    Did you install new rotors? Any chance they are arcing thru the center to distrib shaft? Sort-of quick check is put old ones back in. Even easier if you can identify which bank has the miss.

    Good luck!
     
  16. Stefan Elshout

    Stefan Elshout Karting

    Dec 1, 2011
    146
    Holland
    Thanks for all the advice. As soon as i have time i will do some more checks, also ignition related.
    If I'll be able to trace the problem i will get back here.
     
  17. Andy 308GTB

    Andy 308GTB Formula 3
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    Jun 2, 2004
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    +1

    I rebuilt my carbs last year and I think this describes the problem I had. I wound all the screws in & started again - I ended up making only very small adjustments. The car runs better than ever (and I've owned it since 1998!)


    Sent from my Mi 9T Pro using Tapatalk
     
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  18. TonyL

    TonyL F1 Rookie
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    Taking the time to pre-set the carbs before tuning pays dividends. The crucial bit is to get the throttle plats in the right position with feeler gauges beforehand.
     
  19. TonyL

    TonyL F1 Rookie
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    Stefan - Did you manage to get this sorted?
     
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