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Carb 308 midrange miss

Discussion in 'Technical Q&A' started by BillyD, Nov 15, 2007.

  1. BillyD

    BillyD Formula 3
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    Feb 28, 2004
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    Whenever I was at cruise in my 77 308 I could always feel a miss. I figured I was too rich as the plugs were always black & sooty and I would occasionally foul a plug. Well today after screwing with the points, timing, sparkplug wires & not getting anywhere I put a stick between the gas pedal & seat to hold a steady 2750 rpm where the miss was most noticeable. I ended up turning all the idle mixture screws out approx 2 turns, this puts me 4 to 5 turns out from closed and voila! the miss is gone. With the idles out this far my idle dropped to 500rpm and had to be reset. I have always set the idle mixture screws to the best idle not the best cruise so my fix seems wrong. With the engine cover off & it pouring rain here in the beautiful Northwest I can't get out on the road to test it.
    So any bets whether I cured it?
    Bill
     
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  3. miketuason

    miketuason F1 World Champ
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    Well, if the miss doesn't come back, then I say you fixed it, and good job.
     
  4. Steve King

    Steve King F1 Rookie

    Feb 15, 2001
    4,366
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    Bill just as an FYI my mixtue screws are around 5-7 turns out. I am running NGK BP5ES plugs and have a Pertronix ign. set up running at 7*BTDC. Car runs well up to redline and cruises without any problems. Idle is set at 1000rpm.Idles rich but when I pull the plugs after a road tune up they are pretty clean. Just my set up.
     
  5. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
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    Billy,
    The idle mixture screws only affect the fuel coming from the first progression hole, so it is truly meant to adjust the mixture just at idle. If you are having lean issues in the transition region at 2700 RPM, I would suggest that you need richer idle jets and then actually close down the idle mixture screw to keep the idle mixture right. What you are doing now is attempting to cover up a lean idle jet by compensating with the idle mixture screw. This can only help you so much, as the flow is still limited by the jet itself.

    Birdman
     
  6. chrismorse

    chrismorse Formula 3

    Feb 16, 2004
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    Hi Bill,

    This topic was covered extensively in the following thread in the 308 section http://www.ferrarichat.com/forum/showthread.php?t=164562

    The short answer, if I remember Russ's recommendation was to fatten up the main so it reaches down further. I believe this is preferred over installing larger idles because almost all of the cruising is done on the idles. I went from stock 55s to 60s and my mileage is poo-poo.

    hth,
    chris
     
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  8. Artvonne

    Artvonne F1 Veteran

    Oct 29, 2004
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    Because it was really years since I messed with carbs, you forget things. As Birdman eluded, there are progression holes along the throat where the butterfly sits in the idle position. Depending on the throttle position and engine speed, there will be more of less flow at those holes. Your mixture screw controls the flow of fuel at those holes, and your idle speed is then dependent on both the throttle position (butterfly) and the mixture screw setting. If your mix screws are screwed in to far (lean) you will be forced to open the throttles (butterflies) farther to get enough fuel to idle. In other words, the idling speed is more than just turning the idle speed screws, its a concert of things working in unison. As you open the throttle to increase speed, more holes are exposed for fuel/air mixture to flow from, and its at some particular position it was running out of fuel.

    Other items that will effect idle speed and quality are valve timing and valve clearance, proper carb idle jet and air bleed size, emulsion tube, progression hole sizes and placement, as well as the correct spark plugs and ignition timing. If your turning the screws out more than 4 turns you could probably run a larger idle jet, or reduce the idle air jet size. I dont have any at present, but I want to get a set of F24 emulsion tubes and some .45 and .50 idle jets, as well as check the progession holes before I tinker anymore.
     
  9. Aircon

    Aircon Eight Time F1 World Champ

    Jun 23, 2003
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    how do you do that?
     
  10. Birdman

    Birdman F1 Veteran

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    Paul,
    This is only partially true. On the Webers on 308s, only the FIRST (idle) progression hole is metered through the idle mixture screw. The remaining progression holes are fed directly from the jet. (The diagram in the GT4 service manual shows it clearly. See next post.)

    The point is that adjusting the idle mixture screw is only changing the flow on the first hole. If someone has a lean issue in the transition region (and this is what we are guessing is causing the miss in this case, but we don't know for sure) the only proper solution is a larger jet. In this region, it would be my choice to richen the idle jet and then reduce the idle mixture at the first progression hole with the mixture screw. The other option is to "reach down" into the transition region as Chris suggests, with a large main jet. The only issue here is that you may end up too rich on the top end (which could be tweaked with air correctors). The best way to know which is better is by putting the car on an A/F meter and running it. Or you can do what I do and guess....nto always the best way!

    Of course, there is also the possibility that this is ignition related, but midrange misses are usually carbs (high end misses are usually ignition).

    Birdman
     
  11. Birdman

    Birdman F1 Veteran

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    #9 Birdman, Nov 21, 2007
    Last edited by a moderator: Sep 7, 2017
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  13. snj5

    snj5 F1 World Champ

    Feb 22, 2003
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    Which is WHY there are air correctors. :)

    If you are tweaking carbs a good mobile A/F like an LM-1 really allows you to dial in carbs (or even FI).
    Ask Aircon - he has really done well.
     
  14. Birdman

    Birdman F1 Veteran

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    I wish I knew more about air correctors. I have never diddled with them.
     
  15. snj5

    snj5 F1 World Champ

    Feb 22, 2003
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    #12 snj5, Nov 21, 2007
    Last edited by a moderator: Sep 7, 2017
    Buds,
    It's just as you think, as the larger mains allow the main circuit to come in sooner, at higher flows the a/c act to put a brake on the suction of the venturi, leaning the mixture. So you can dial in leanness as the mains progress by using larger air corrections.

    As a made up example, let's just say that we have a small flat spot using 130/F36/190/55 at about 3000 rpm with the stock 32mm venturis. So, it would be silly to 'reach up' by increasing the already relatively large 55 idles, so we 'reach down' to cover the lean hole by bringing in the mains sooner with the 135. So, if with the 135 the top end becomes too rich (which is a discussion in itself as to what constitutes too rich in the high register), we'll say an A/F of 11 at 5500 is what we see on an A/F meter (e.g. LM-1). Here we would brake the suction on the main using a larger a/c to "short out" the suction applied by the venturi to the main jet and thereby leaning out the main jet as the rpms increase. A rule of thumb I seem to remember is two sizes A/C to one size main, but would rely rather on an A/F meter. So, we keep the same cruise mixtures, cover the hole, and keep acceptable high register A/F with a possible combo of 135/F36/200/55.

    Obviously, this is tweaked with an A/F meter.

    In fact, you might be able to back off the idle to 53 if the main reaches down far enough, with resulting better mpg and less plug fouling tendencies. This I've found can be done less empirically and is more trial and error when reducing idle jet size. Although I have NEVER seen a whole lot of leanness from Webers in the idle circuit, I'd caution against taking much leaning without an A/F meter. You are also somewhat safe down in the low rpm as typically the car is NOT under load and is more tolerant of lean mixtures, and you have the accell pumps to cover A/F on any wanton right foot misadventures. :)

    For my next trick, I'll explain ET tube function... well, maybe not. :)

    YMMV
    And Happy Thanksgiving to all.
    rt

    Reference - How to change jetting:
    http://ferrarichat.com/forum/showthread.php?t=42363
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  16. Artvonne

    Artvonne F1 Veteran

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    Yes, your correct, my appologies. Sometimes what we mean to say and how we say it gets lost in the translating.

    As to changing the idle air jet, you could simply drill it larger, or pull it out and replace it with a smaller bushing.

    Russ, thanks for the pics of the two emulsion tubes. Not only is the F24 smaller in diameter, it also appears to have much larger holes at the bottom. Quite a drastic change compared to the F36 tube.
     
  17. Aircon

    Aircon Eight Time F1 World Champ

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    thanks!

    couldn't be happier

    http://www.fcavic.com.au//308/exhaust.htm
     
  18. Birdman

    Birdman F1 Veteran

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    Russ,
    Thanks for the example. One of these days I will get an A/F meter and really dial in my car. For now, it runs great, even though it's a little rich and gets bad milage!

    Birdman
     
  19. BillyD

    BillyD Formula 3
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    I was searching threads for info on the LM-1 meter when I saw this old thread & figured I'd update it. The miss was caused by bad plug wires. I still have a stumble at light throttle in the 3-4k rpm range especially in corners. I want the LM-1 to see the air/fuel ratio at these times. Any comments on the LM-1? Does it connect to the 308s tach ok? Do I need a bung or out exhaust pipe good enough?
    thanks
    Bill
     
  20. snj5

    snj5 F1 World Champ

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    #17 snj5, Jun 26, 2008
    Last edited: Jun 26, 2008
    I really like the LM1. An exhaust sensor may be ok if you can place the sensor far enough back, but a bung as near to the header collector is most optimal. Any local muffler shop should be able to install it for about $20, and each LM-1 comes with one in the kit.
    One thing some folks may want to remember is that many carb cars have essetially separate exhausts per bank. That is to say, to diagnose a single carb fault, you may need a bung on both sides of the exhaust. For just setting the jetting, if the car is otherwise good, one side should basically reflect the other.

    The basic LM-1 one does not come with a tach pick-up- a coil wire pick-up kit must be purchased extra. I just got one.

    Stumble in hard corners is sometimes a feature rather than a problem with the DCNF, however it can be minimized by ensuring float levels are set well. Although I have not tried it, some folks get good results from raising the float level. The 3-4K miss could also be a more classic transition lean spot, or a combination of both.

    I will proselytize for a moment with my usual sermon that if you are still running the original stock jetting, it is probably not optimum on today's gasoline formulations when compared to the 70s when the Webers were initially jetted. Newer gas usually needs a bit richer main, and thankfully Webers are so very easy to re-jet.

    Hope this helps,
    best,

    rt
     
  21. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
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    Hey Bill, I had a miss recently caused by a bad connection on a plug wire and it sometimes feels and sounds very similar to a carb issue. Glad you found it!

    Birdman
     

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