308 Carburetor popping issue | Page 3 | FerrariChat

308 Carburetor popping issue

Discussion in 'Technical Q&A' started by Peter, Sep 2, 2019.

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  1. absostone

    absostone F1 Veteran
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    Jul 28, 2008
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    Ngk plugs lower number equals hotter
     
  2. absostone

    absostone F1 Veteran
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    Jul 28, 2008
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    Also the 2 videos posted on this thread are showing idling carbs at 5KGH/ as are mine also. Though they are supposed to be at 3.5 to 4.5 KGH. Some one should post a vid at 3.5 KGH
     
  3. wrxmike

    wrxmike Moderator
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    Mar 20, 2004
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    Mike
    Yes. The 'factory" recommendation is a -7 or - 8
    More important is that the airflow is the same across the carbs, (balanced) and that when you crack open the throttle, the balance is maintained ( ie no slop in throttle linkages).
     
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  4. 2dinos

    2dinos F1 Rookie

    Jan 13, 2007
    2,772
    Where does it show flow rate at idle specs? I just use the data to compare the flows to each other, and don't care beyond that.

    If a cam belt skipped a tooth, that would make carb setup a lot of fun. I think the flow rates would be wildly off if it were cam timing.
     
  5. Peter

    Peter F1 Veteran
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    Dec 21, 2000
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    I haven't tried that yet. I got a set of 7mm spark plug wires today and am in the process of installing them (I forgot just how fun of a job that is...).

    I have taken all of that into consideration. I too thought it may be fuel pressure/volume related and can still check that, but if there's a problem at idle, there'd definitely be serious problems under full demand and that wasn't the case. I am running six-range NGK plugs BTW.

    I don't think that's possible. There is a marked difference in idle speed even in half kg steps. I don't think the engine would run if it were that low (or at least not very well).

    Cams did not skip a tooth. All of the marks are spot-on to the bearing caps.

    It will be a busy weekend and I will try to carry on testing...
     
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  6. absostone

    absostone F1 Veteran
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    Jul 28, 2008
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    I’ts been said all over fchat 3.5-4.5kgh
    I’ve haven’t been able to achieve that.
     
    Peter likes this.
  7. afwrench

    afwrench Formula Junior

    Nov 24, 2004
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    Mike
    Peter, please do things in a systematic way.Change only one thing at a time so as to not confuse one issue with another.There is always a reason for what is happening. Persist and you will find it . Good luck,Mike
     
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  8. Peter

    Peter F1 Veteran
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    Yes, I am trying to do that to the best of the given time I have to dedicate to this job.
     
  9. Ak Jim

    Ak Jim F1 Veteran
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    Dec 23, 2007
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    North Pole AK
    Copper core wires! All I can say is OMG! I had an air cooled VW in the early 80s. The engine was pretty stout. High compression, pretty aggressive cam and dual 44 Webers. In an effort to try to get the most performance from the engine I got a set of copper core spark plug wires. The interference with the radio was unbelievably bad. A friend and I did some informal tests. He would park his car and I would drive by. If he had his AM radio on and not tuned to a station he could start to hear the ignition noise when I was 4 blocks away! When tuned to a station he could start to hear the noise at 2 blocks way. It was crazy. Maybe it was just my imagination but the engine did run better with the copper wires but it was just too much interference. I ended up using the copper wire for the coil wire and regular wires for the individual plugs.
     
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  10. Peter

    Peter F1 Veteran
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    In today's technological, connected world, if I installed those wires, I think the cars beside me along the road would start shutting down! Or crash into me!

    Jokes aside, as mentioned earlier, the Pertronix ignition modules cannot be used with copper wires... And I'm not going back to points.
     
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  11. afwrench

    afwrench Formula Junior

    Nov 24, 2004
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    Hi Peter, any progress? Mike
     
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  12. Peter

    Peter F1 Veteran
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    Thanks for your concern and just by coincidence, I was going to post this evening.

    So, I finally got around to doing a leak-down test. Back in 2012, I tested my car as my good friend Rob Garven wanted to get some confirmation when he was working on his car. We both had the same testers and we both used the same baseline pressure (80 PSI). Engine was stone-cold when performing tests then and now. I kept that info and referred to it for this. So the results are below:

    March 2012 Sept. 2019

    1- 79 PSI 1- 78 PSI
    2- 79 PSI 2- 79 PSI
    3- 79 PSI 3- 79 PSI
    4- 79 PSI 4- 78 PSI
    5- 78 PSI 5- 78 PSI
    6- 78 PSI 6- 78 PSI
    7- 78 PSI 7- 78 PSI
    8- 79 PSI 8- 79 PSI

    So, after 7 years and thousands of miles later, THERE IS VIRTUALLY NO CHANGE IN RESULTS. And, considering the engine was stone cold and would be in it's loosest, leakiest condition, a drop of 2 PSI at the most and I think I should consider this healthy. Keep in mind, the problem cylinders I have (7 & 8), have not changed. I made notes of where I heard leaking coming from and that hasn't changed either. In fact, the cylinder that's popping the most on me, #7, I don't hear any air escaping from either of it's valves (you can hear it whistling past the ring gaps). No air leaking into the coolant system either.

    I'm kind of ruling out a mechanical issue based on this.

    Carrying on...

    Image Unavailable, Please Login
     
  13. 2dinos

    2dinos F1 Rookie

    Jan 13, 2007
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    Been away for a bit. Does it run happy now?

    One thing I just thought worth mention. That weird clog in a seemingly insignificant passage was only a problem in one carb on one side. Point being, this 'could' be a problem if you notice just a one, maybe two spots not acting right. Who wants to disassemble what isn't necessary.
     
  14. 2dinos

    2dinos F1 Rookie

    Jan 13, 2007
    2,772
    Just curious about the ignition. Was there a time before working on the carbs where it ran well?

    You mentioned having it set to 7 degrees BTDC, but was the timing perfectly consistent? Did you see the ignition fire with a timing light? Could it be a bad ground or whatever? I have seen an impossible to set up car which had a subtle ignition inconsistency. It had points, and the dwell was too small on one of the points.
     
  15. Peter

    Peter F1 Veteran
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    I haven't had a chance to run the car.

    Yes, it did run well before this popping issue came about and yes, both banks are set up to 7 degrees BTDC. I'm not running points anymore, I have the Pertronix dual triggers-in-one-distributor set up.
     
  16. spicedriver

    spicedriver F1 Rookie

    Feb 1, 2011
    3,859
    There was a previous post about the possibility of an air leak at the carb gasket. This can happen especially when the carbs are removed for cleaning. I would apply a silicone sealant to both sides of the carb gaskets, and see if that fixes your problem.
     
  17. Andy 355

    Andy 355 Formula Junior

    Jun 2, 2013
    434
    Sydney
    Crazy... but.. you have checked the plug leads are on the correct plug. Firing order?
     
  18. TonyL

    TonyL F1 Rookie

    Sep 27, 2007
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    Tony
    With the DCNF webers it is critical that you get the airflow thru each carb exactly the same + some other parameters checked. Therefore a pre-setup is needed.

    IMHO, 5k is way to high and will probably lead to the first progression port being exposed thus rendering the mixture screw totally or partially unresponsive. Ie. it bypasses the idle circuit and allows fuel to spill into the barrel at an uncontrollable rate.

    The optimum flow rate should be around 3.5k @900rpm max

    Improperly set carb linkage can cause snapping out of the throat or badly worn linkages and heim joints. Any slop in this area and you are wasting your time, take the trouble to remove them and check over thoroughly

    I would disconnect all the linkages, wind out the idle stop screws and reset so a 0.004 gap is obtained with a feeler gauge between all the diffuser walls and butterfly, set each one so they are all the same. Then reconnect the linkages trying not to disturb the pre setup. It may run fast but you can alter them later using the flow gauge. check the air bypass screws are max 1/2 out or preferably closed. (if they are to far open then they can cause a lean condition in the idle circuit) The air bypass screws can be altered once you dial in the flow rates to each barrel

    Check the float valve actually operates and shuts off, if the fuel pressure is too high (4psi max) then fuel can spill over into the barrel and cause the snapping.

    Vac leak can cause this as well.

    Once you have done the pre set up then balance carbs to get 3.5kg at 900rpm, then adjust mixture screws

    Hope this helps.

    Tony
     
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  19. Peter

    Peter F1 Veteran
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    I always use Hylomar and yes, I have a wee bit on them.
     
  20. Peter

    Peter F1 Veteran
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    Yes.
     
  21. spicedriver

    spicedriver F1 Rookie

    Feb 1, 2011
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    If a valve stuck open on this motor, it would get hit by the piston and become bent. Then you wouldn't be getting a good compression reading.

    Does the car have new plug wires yet ? This is a common fault. You might be able to swap a wire from another car into the #7 to test.
     
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  22. Peter

    Peter F1 Veteran
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    Yes, I finished installing a new set of wires this past weekend. I hope to get the car out this coming weekend...
     
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  23. BigTex

    BigTex Seven Time F1 World Champ
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    Dec 6, 2002
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    NGK 5s here, anything colder warms up too slow, in the tropics!!
    Of course it's spinning 20 -50W

    But there's a sweet gap, a little past.20....I shoot for .22.

    Some of the new 5-50W are closer to AGIP 10- 50W IMO.
    But, this is the "misfire" thread!!
    :D :D :D
     
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  24. absostone

    absostone F1 Veteran
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    Jul 28, 2008
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    stock ignition with points?
     
  25. f355spider

    f355spider F1 World Champ
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    Must be with that small a gap
     

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