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Weber questions?

Discussion in 'Technical Q&A' started by Doug, Mar 20, 2004.

  1. Doug

    Doug Formula 3

    Nov 13, 2003
    1,467
    Louisville KY
    Full Name:
    Doug
    Picked up a '76 308 GT4 with dcnf 40's. Carbs may just need a complete rebuild but I thaught I would first post for potential fixes. Car has single dist. elec. conversion and I installed new plugs gapped at .36 before the 100 mile drive home.
    Car starts and idles great.
    Hesitates a little at low rpm when first taking off.
    Mid range is great. 65-80 in 5th ran flawless. I can't state the rpm because the tach is disconnected because it reads double since the single dist. comversion. My guess is 2500-4000 in all gears is pretty good.
    Bogs down at higher RPM in any gear and pops/backfires. This is probably above 5000 rpm. Not sure if that means too lean or rich?

    Also, when coming off throttle and engageing the clutch for a gear change up or down, the rpm doesn't immediately drop, it is quite slow to come back down. Engine braking will bring it down.
    Any thoughts or suggestions from you Weber experts? Just thought I would see if I can do any adjustments before I decide to send them out for a rebuild.

    BTW, I removed the air box to see if anything was binding to cause the high rpm when shifting and could not see anything being held up. The linkage would spring back but the rpms still dropped slowly.
     
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  3. pma1010

    pma1010 F1 Rookie

    Jul 21, 2002
    2,559
    Chicago
    Full Name:
    Philip
    Doug
    Welcome to Webers. I've just rebuilt mine top to bottom. Recommend you get out your parts book, look at the Pierce manual and buy all the wear parts.

    On the specifics:
    the slow return to idle can be sticking throttle shafts. Luckily the 40 DCNFs have ball bearings so you can clean out and lube when you have them apart.
    the backfiring through the ex pipe indicates a rich condition. Might be restrictive air filter, wrong jets or leaking accel diaphragms.
    spitting up through the venturis means its lean.
    Above 5800 or so, you are on the air corrector jet...
    best plan is to pull them off, give them a good overhaul and check the jets versus spec.

    Have fun!
     
  4. dm_n_stuff

    dm_n_stuff Global Moderator
    Global Moderator Lifetime Rossa Owner

    Dec 10, 2003
    36,990
    Babcock Ranch, FL
    Full Name:
    Dave
    Save some aggravation and buy a rebuild kit. I'm guessing Ferrari UK has one, and I know that www.Superperformance.co.uk has one, they actually have a discount for the 4 carb kit.

    Then pick up a good weber book, and have at it.
     
  5. yelcab

    yelcab F1 World Champ
    Consultant

    Nov 29, 2001
    10,718
    San Carlos, CA
    Full Name:
    Mitchell Le
    I would say, don't try adjusting them. Rebuild them first, then you will start at a known point. Also, make sure the engine is in proper working conditions before you attemp the carbs rebuild.
     
  6. ham308

    ham308 Formula Junior

    Nov 3, 2003
    358
    NE Switzerland
    Full Name:
    Richard Ham
    If the ignition system has been changed it would be an idea to check the spark advance with a strobe light first before starting on the Webers. The value is in the manual, but it's something like 30 deg advance at 3400 rpm.

    It's an easy thing to do and could well be responsible for the poor high rpm performance.

    good luck
     
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  8. pma1010

    pma1010 F1 Rookie

    Jul 21, 2002
    2,559
    Chicago
    Full Name:
    Philip
    34 @ 5000 rpm
     
  9. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
    2,279
    Roseburg, OR
    Full Name:
    Hans E. Hansen
    Try George Evans for the rebuild kits. He sells a kit that does all 4.

    BTW, you can also get exhaust pops/backfires with too lean. The lean mixture doesn't ignite in the cylinder, but can and will ignite in the muffler.
     

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