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Boxer flat spot on accelerating

Discussion in 'Boxers/TR/M' started by steveirl, Nov 11, 2008.

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  1. steveirl

    steveirl Formula 3

    Dec 29, 2003
    1,163
    ireland
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    steve
    #1 steveirl, Nov 11, 2008
    I collected my Boxer fron the UK last week. Bit of backround first, its a 78 BB512, which I bought in the summer of 2006, very nice condition, 18k miles, but it has been in storage for most of the last fifteen years. After buying it I left it in storage until Jan this year when I instructed the specialist who sourced it for me to begin service and recommissioning. They done a full engine out service, belts, valve clearance, brake overhaul, replaced all hoses, new ignition module(aftermarket) distributor overhaul, stripped and cleaned the carbs, shocks rebuilt. I had intended to bring it back home shortly after this was finished in July(as the car qualifies for tax exemption at 30 years) but could not do so until last week, so the car was once again stored for the last 6 months.
    It has a pretty bad stumble at about 2000rpm when accelerating, but you can drive around it by easy throttle openings until about 2500rpm, then it flies, pulls like a train. The flat spot does not seem to be as bad when the car is cold, and when it happens there is a cloud of black smoke from the exhaust. The car starts and idles perfectly both hot and cold. I called the specialist to ask his opinion, and he said he had checked the dist. advance and it was fine. He advised looking at what he called the "backflow jets" in the bottom of the float chambers, and if they are numbered to replace them with 0 jets, saying this sometimes helps.
    His take is that the flat spot can be driven through, and sorting it may take a lot of time, and I should enjoy the car and not worry unless it gets any worse! Needless to say its not what I wanted to hear.
    I am no mechanic, but have maintained to a fair degree my cars with help from friends, and owners forums like here. So any ideas on how i should troubleshoot would be great.
    Steve
     
  2. JTR

    JTR Formula 3
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    #2 JTR, Nov 11, 2008
    Last edited: Nov 11, 2008
  3. 512bbnevada

    512bbnevada Formula Junior
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    #3 512bbnevada, Nov 11, 2008
    check your carb linkages to see if they are all in sync, transistion one carb may be opening a bit more than another etc., this IMO takes someone who really knows how to tune a 12cylinder carb Ferrari
     
  4. jselevan

    jselevan Formula 3

    Nov 2, 2003
    1,873
    #4 jselevan, Nov 11, 2008
    If you can live with it, drive and enjoy until something else occurs that requires attention.

    Webers are known to have this characteristic stumble at 2,000 to 2,500 RPM. My Dinos have it, my C4 has it, etc. My Alpine-Renault with Webers has it. It represents air/fuel flow during the transition from idle jets to high-speed jets.

    There are work-arounds, and Weber even had a update that required drilling a small hole in a butterfly valve. A perfectly tuned Weber can be made to correct this, but it does take a keen ear and a fair amount time and interest (passion).

    Have others resolved this stumble at transition RPM?

    Jim S.
     
  5. JazzyO

    JazzyO F1 World Champ

    Jan 14, 2007
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    #5 JazzyO, Nov 11, 2008
    To add to jselevan's comment: my 365 Boxer also hesitates when it switches from the idle to high speed jets. All carb Boxers do it to some degree, and it is indeed notoriously difficult to get rid off. There are some people that have a knack for it and they still require a lot of time to get it right.

    The smoke you see might just be dumping of fuel (or is it really black, not blue?) as it to high speed. Mine switches at 3,500rpm or so, though - it doesn't hesitate at 2,000.


    Onno
     
  6. fastradio

    fastradio F1 Rookie
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    Apr 26, 2006
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    #6 fastradio, Nov 11, 2008
    Steve,

    Please see post #22.
    http://www.ferrarichat.com/forum/showthread.php?t=210154&page=2

    If your slow speed mixture screws are "out" more than 4 turns, replace the idle/slow speed jets with .55mm, verus the .50mm original equipment jets. This change alone will greatly reduce the transition stumble. Also, further improvements can be had by raising the float level about 1mm. As mentioned before by others, synchronization is also critical. The carbs should be initially synched at idle, with the cross-engine linkage disconnected. However, the synchronization must also be checked at say 2000-3000 rpm (steady), with all of the linkage rods in place. This "high rpm" check is very important, as on "tip in", any wear in the linkage will now become apparent (resulting in a possible stumble). I use two tools for the carb set-up; A single barrel "synchrometer" for the base settings...and either a multi-tube manometer, or a pair of precision matched vacuum gauges for the high rpm check.

    However....as JimS said so elegantly, this "stumble" really is inherent to the Weber design...and can be extremely difficult to eliminate 100%. With patience, 98% can be "tuned out"....


    Regards,
    David
     
  7. Murph

    Murph Formula 3

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    #7 Murph, Nov 11, 2008
    Cue Newman with an injected joke any minute now...
     
  8. JTR

    JTR Formula 3
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    #8 JTR, Nov 11, 2008
    Wow! 15 minutes and no newman,,,, the guy must be sick!
     
  9. snj5

    snj5 F1 World Champ

    Feb 22, 2003
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    #9 snj5, Nov 11, 2008
    Last edited: Nov 11, 2008
    Many cars have developed this stumble over time not so much to the factory jetting being inadequate, but remember they were jetted for 1970's fuel - wildly different than the stuff we have today. Just speaking for the Weber DCNF crowd, many mixture stumble symptoms that are often flippantly thought to be a "feature" are rather easily tuned out: by increasing the idle jet size as the honorable Fastradio has suggested, or in combination with a larger main. While indeed there is no substitute for a good synch, equally true is that if you are running the initial jetting from the 70s , it's probably just wrong for today's fuel.

    The backflow jets your guy mentions probably refer to the accel pump system, which in your car balance the amount of the pump shot. Going to a 0 size would richen your accel pump shot, covering any transient lean spot, so not a bad idea.

    I'll leave the details of Boxer carbs to David, as he is the Bagwan on those jewels, but these are common issues amoung the DCNF carb'd Ferraris.

    Good luck!
     
  10. steveirl

    steveirl Formula 3

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    #10 steveirl, Nov 13, 2008
    Thanks for the replies, very very helpful as usual. Right now I am detailing the car, but also working 13 hour days 6 days per week so I will probably not have too much time to investigate this. I have ordered a few books on Webers, the haynes manual and Pat Bradens book. I will check the dist. advance and plugs first, and then look for air leaks, before even looking at the carbs. As it is however I can drive it, just have to stay above 3000rpm me thinks :)
     
  11. Modeler

    Modeler F1 Veteran

    May 19, 2008
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    a.n.other
    #11 Modeler, Nov 13, 2008
    Someone who knows webers should get it to be a lot smoother through that first transition for you.
    The whole point of them is how tunable they are.
     

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