Synchronizing carbs

Discussion in 'Technical Q&A' started by Barnone 308, Jun 21, 2009.

  1. Barnone 308

    Barnone 308 Formula Junior

    Mar 22, 2004
    I was synchronizing the carbs on my 78 308 gts, and could not get the back two carbs to synchronize. At 1500 RPM, the back left carb has a reading of 10 on the synchrometer, and the back right carb has a reading of 5. Both front carbs have a reading at a little over 7 on the synchrometer which is good. How does someone fix this problem, is it time for a carb rebuild.
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  3. gcmerak

    gcmerak Formula 3

    Mar 17, 2008
    Engine Bay, Georgia
    Full Name:
    George C.
    My suggestion would be as follows: Make sure you have a reliable tachometer [not the one in the car] that you can read in the engine bay. Turn in the idle speed screw to bring up the numbers on the synchrometer so it is in line with the other carbs. I am assuming that each barrel per carb is pulling the same vacuum and that at least one air bypass screw per carb is fully shut. When you turn up the idle screw the RPMs should barely move yet it should bring the numbers up on the synchrometer. Turn out the idle screw on the carb reading high, but monitor the RPMs on the tach. On my car the RPMs are supposed to be @ 1000.

    If that doesn't do it, it may be time 4 a rebuild. You could be pulling unwanted air through the throttle shaft bearings. Hope this helps.

  4. 2NA

    2NA F1 World Champ
    Professional Ferrari Technician Consultant Owner

    Dec 29, 2006
    Twin Cities
    Full Name:
    Tim Keseluk
    Try not to make this harder than it should be. Carb synch is really nothing more than comparing throttle openings. If one is a 10 and one is a 5, the 10 is doing more "work" than the other. Close down or open up one to get them even. Then compare to the other two.

    Some times you go around a few times before you find the right combination that runs the same number.

    Whether or not your carbs need to be rebuilt, you should be able to get them to pull evenly unless you have a major vacuum leak somewhere.
  5. CliffBeer

    CliffBeer Formula 3

    Apr 3, 2005
    Seattle, Washington
    Full Name:
    If you're not working off the idle circuit (ie. rpm is 1,500 or above, roughly) and thus the butterflies are just slightly cracked open then you're challenge is to simple ensure the relative position of the them is the same. If you're getting different flow readings then in all likelihood the butterflies are not opening and progressing in sync. Look to see if you have slack/slop in the linkages - this can cause one or more butterflies to open earlier, and more, than the others. The adjustment is most critical just when the butterflies are first moving off closed (and the carb is transitioning from the idle circuit to the progression circuit). At WOT it doesn't matter a whole lot if one butterfly is a couple of degrees more progressed.
  6. spider348

    spider348 Formula 3
    Silver Subscribed

    Nov 3, 2003
    Full Name:
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  8. Barnone 308

    Barnone 308 Formula Junior

    Mar 22, 2004
    I appreciate everyones help. I actually am using birdmans procedures to do this. Since I am only synchronizing the carbs right now to make sure they have the same flow, all my air bypass screws are in, and I will begin balancing each carb after I have them in synch at 1500 rpm. The procedure is to first have the sucking the same air at 1500 rpm, and then droping the idle screw to 900 rpm, and adjusting air bypass screw and mixture at that point. I did have to pull put the mixture approximately four turns because I had popping when I first started the car and set it at 1500 RPM. I am a little confused with something. What screw moved the butterfly valves in the car, Is it the one screw that is used to synch the carbs, I know birdman is pretty good, and his explanation is that turning the one screw you can sycnchronize the back carbs, and use the other synch screw to synchronize the front. When turning the sych screw does that impact the butterly valves ( I thought it was the air bypass screw) , I have a hard time believing that turning one screw synchronizes the carbs. because I have turned that screw, and it does not sychro the carbs. I have no air leaks, I replaced and clapped all the hoses. Birdman suggest that the airbypass screws should only be used to sychro each individual carbs barrel to each other, and not to other carbs, so gcmerak, having one bypass screw fully opened does not make sense at this point according to birdmans instructions, and assuming that birdman is right. 2NA, I am not sure what you mean by just opening up, and closing down one to get them even, what is the procedure?. Cliff, I agree with you about the linkages that can possibly be a factor, I will need to take a look tonight. I appreciate everyones help.
  9. tuttebenne

    tuttebenne F1 Rookie

    Mar 26, 2003
    Bay Shore, NY
    Full Name:

    Is all the linkage off?

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