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Carb question

Discussion in '308/328' started by Doug, Jun 14, 2009.

  1. Doug

    Doug Formula 3

    Nov 13, 2003
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    Doug
    I have been searching the archives but have not found the answer I am looking for. My '77 runs great except when first taking off under load. It momentarily bogs down and stumbles off idle and then everything is great up to redline. I doesn't ever stall out. The car has Electromotive ignition. Is this more of an adjustment to the carbs, or is it probably due to jet sizes?
     
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  3. mike996

    mike996 F1 Veteran

    Jun 14, 2008
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    I don't know the first thing about the Weber carbs as used on these cars but I have a LOT of experience with other carbs. The symptom you describe is typical of excessive leanness as you apply the throttle and is usually caused by a faulty or misadjusted accelerator pump(s). It has nothing to do with ignition or jetting.
     
  4. Newman

    Newman F1 World Champ
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    I was having a similar problem. Im upping my Emulsion tubes to F24's from F36's to richen things up a bit and cover the lean spot at transition.
     
  5. thecarreaper

    thecarreaper F1 World Champ
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    Sep 30, 2003
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    agree. 135 main, 50 idle, F36 em has been a great combo for me with NGK BP5 or BP6 plugs in the 4 carb cars i have owned.
     
  6. fastradio

    fastradio F1 Rookie
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    Apr 26, 2006
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    I've worked with Weber carbs on Ferrari's, Porsche's and Alfa Romeo's for just over 25 years now...so here's some things I'd be looking at.

    Based on your symptom "It momentarily bogs down and stumbles off idle"...and runs fine, other wise, I'd start here:

    -Verify that your static ignition timing is correct
    -Verify that your ignition is advancing
    -Verify that your idle jets are .55mm versus the .50mm jets that the car likely came with
    -Go through a complete carb set-up, verifying that the "air bleed" screws are either all closed, or are within 2 turns of closed
    -Set idle mixture, either with an exhaust gas analyzer or using a "lean drop" procedure
    -Off idle performance is directly linked to your idle synchronization-Verify that your carbs are "in-synch" both at and off idle by disconnecting the carb linkages when setting the initial synch...and then re-verify that that stay in synch when the linkage is connected using a pair (or multiple pairs of matched high-precision vacuum gauges) or a multi-tube mannometer.

    The accelerator pump circuit doen't not come into play directly off idle. The pump circuit is important during the transition phase, which typically occurs between 2,800-4,200 on a Weber.

    If your car stumbles during the transition phase, i.e. when going from the idle circuit (which in a typical Weber carb) is also the low speed circuit, all of the above applies as well, but the emulsion tube selection now comes into play.

    The ignition circuit and all the related components should never be discounted with any vehicle which has a driveability concern. Ignition problems, as well as carb synchronization problems can be masked at higher engine speeds.

    David
     
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  8. Beta Scorpion

    Beta Scorpion Formula 3

    Jun 22, 2006
    1,190
    77s ran fine when new so changing the emulsion tubes and jets isn't needed if everything else is OK (and your car has not been modified).
     
  9. fastradio

    fastradio F1 Rookie
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    That's true...but the fuels today with typically 10% ethanol, no lead and MTBE, will cause rejetting for optimal performance in most cars of that era.
     
  10. mike996

    mike996 F1 Veteran

    Jun 14, 2008
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    So a Weber acceleration pump doesn't have any effect from idle? Seems a strange method of operation but I certainly learned something today!! Guess I better stick to carbs that I know...with names like Holley, Edelbrock, Carter, etc... :)
     
  11. Newman

    Newman F1 World Champ
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    Like David said, fuel formulation is different now than in 1977. Oxygenated fuels means carb changes are needed.

    Yes, different than a holley, the ramps are quite far from the pump levers at idle which being being raised on holleys threw me for a loop too.
     
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  13. fastradio

    fastradio F1 Rookie
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    Interesting perspectives, guys....as I've never worked on anything but single-barrel Weber's.
     
  14. Newman

    Newman F1 World Champ
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    Take a trio of holleys for a 440 sixpack, no air bleed screws, sync mechanically and adjust idle mixture. There is a trick to setting the mixture screws on the outboard carbs but easy once you know how. They dump so much fuel through the engine a lean spot could never happen. Hemi, just as easy with 2 fours. The webers are cool but seem over complicated when considering how easy multi-carb set-ups are on domestic muscle cars and they never have finicky moments that depend on the alignment of the planets to work right.
     
  15. snj5

    snj5 F1 World Champ

    Feb 22, 2003
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    Webers are very intuitive....but, you must think in Italian.

    Assuming your ignition is correct, and you have tried the stuff below, a lean spot just off idle which goes away at high flows could also be a small vacuum leak. This could come from old hoses...or, if your carbs have never been rebuilt, a leaky shaft bearing.

    But, if your carbs are carrying the original jetting, they definitely need newer and richer jets.
     
  16. CliffBeer

    CliffBeer Formula 3

    Apr 3, 2005
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    Doug, I'd encourage you to get a copy of "Weber Carburetors" by Pat Braden. This book will give you everything you need to fully understand the complete workings of your webers and diagnose any carb related issues. Very, very helpful book.
     
  17. Rifledriver

    Rifledriver Two Time F1 World Champ

    Apr 29, 2004
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    American multicarb applications almost never have a barrel dedicated to a single cylinder. With very few exceptions one primary barrel runs 4 cylinders. That is no different than a single carb application for most of the adjustments. By the time you are into the secondaries or the second and or third carb jetting can certainly make important differences but but adjustments are not nearly as critical. By then it is much like flushing the toilet. When you have a carb throat dedicated to each cylinder it is a very different ball game.

    In my youth I worked on many multicarb American V8s and never understood what all the fuss was about in tuning multicarb motors until I started working on Italian cars.
     
  18. Beta Scorpion

    Beta Scorpion Formula 3

    Jun 22, 2006
    1,190
    True, though it seemed the OP had much, much less then optimum performance. Our 77 and 78 drive fine and don't bog like that with stock jetting and contemporary fuel (but I'll agree they probably could be re-jetted for optimum dyno performance).
     

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