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Carbs to be rebuilt in the Spring

Discussion in 'Technical Q&A' started by jpl, Dec 17, 2003.

  1. jpl

    jpl Formula Junior

    Dec 4, 2003
    314
    Naples
    Full Name:
    JP Lavigne
    I am planning to have the carbs rebuilt next spring on my 78 308 GTB. The dealer quoted me a cost of about $250 per carb. As I don't know much about carbs, other than the fact that they can be "tweaked" for more HP, I am now open to suggestions from the board on some simple tweaks that I could ask the dealer to do during the rebuild.

    What I am looking for is ideas and suggestions on getting a little more HP out of my existing setup during the carb rebuilds. Do I suggest bigger jets......? Open to suggestions. Apox costs and extra HP expected would be nice. Thanks in advance.

    Regards,

    JP
     
  2. f355spider

    f355spider F1 World Champ
    Rossa Subscribed Owner

    May 29, 2001
    16,553
    USA
    Full Name:
    Hugh G. Rection
    USA car? On my 78 308 GTS, with fresh major service, K&N air filter, and test pipes in place of cats; I had a dyno shop change the main jets from 125 (stock) to 130, then 135 and finally 140, to optimize the air/fuel ratio. It was still very slightly lean, but he was afraid the next bump would put us over. Gained about 16 hp or so....final correct hp at the crank was 214hp. Stock (usa) 78-79 308 is 205 hp. Obviously my car was not making even stock 205 hp because the K&N was causing an excessively lean situation. Changing jets is a 10 minute procedure.

    BTW, with the richer jetting, you may need to go to a hotter plug. I was using NGK 6's, but 5's might have been better.

    Oh, and $250 per carb for rebuild is a rather excessive charge. I think my independant Ferrari tech charged me about $500 for all four, including all the final set up and adjustments needed.
     
  3. 4Webers

    4Webers Formula Junior

    Nov 12, 2003
    276
    Texas
    Full Name:
    Darrell
    jpl,
    there was a thread a few days back where a couple of FChatters were discussing sending their carbs to Pierce Manifolds for service. You would have to take the carbs off yourself though (not exactly simple, but definitely do-able - you'll spend 80% of the time looking for the right wrench to get at the nut under the accel pump).

    davehanda,
    Did you change your air correction jets any, or was the K&N the 'air correction'? Now that my '79 308 is finally old enough to be exempt from emission tests here, I am going to start trying to regain a few 'lost' HP myself.

    thanks
     
  4. Mike328

    Mike328 F1 Rookie

    Oct 19, 2002
    2,655
    Boulder, CO
    Full Name:
    Mike
    My carbs are at Pierce Manifolds now, currently "in process." Very pleased with their service.

    Note that they seem to be on an "appointment" basis, and I was lucky to have my carbs squeezed in due to a "cancellation!" My impression was that they had a fair amount of demand and that otherwise their first opening was April...

    Taking off the carbs was the coolest thing I had done to date with the car. Was a lot of fun, actually. You'll make it less painful by just GOING TO SEARS and buying a Craftsman Professional ("polished") stubby 13mm wrench. A little penetrant sprayed on each stub helped out a fair amount.
     
  5. jpl

    jpl Formula Junior

    Dec 4, 2003
    314
    Naples
    Full Name:
    JP Lavigne
    I am not sure what you mean by air correction jets, as all of the air-pump stuff was removed at the dealer when I bought it several years ago. As for taking the carbs off myself.....I wouldn't even attempt it. I can change oil and change a spare, but not much else.

    The dealer did say however a year or so back that the car was producing about 220+hp at the rear wheels, so I am just looking for a subtle change as I am spending the money to re-do the carbs anyways.

    So far the larger jets make the most sense. Can I assume these are changed during a re-build?
     
  6. cavallo_nero

    cavallo_nero Formula 3

    Nov 3, 2003
    1,606
    colorado
    Full Name:
    Giovanni Pasquale
    rejetting the carbs for mo power should only be done on the dyno so they can monitor A/F ratio and HP.
     
  7. 4Webers

    4Webers Formula Junior

    Nov 12, 2003
    276
    Texas
    Full Name:
    Darrell
    The air correction jets are part of the carb, they control the amount of air allowed into the emulsion tubes to mix with the fuel amount, which is controlled by the main jets.
     
  8. Mike328

    Mike328 F1 Rookie

    Oct 19, 2002
    2,655
    Boulder, CO
    Full Name:
    Mike
    Also note the air correction jets impact the mixture most significantly in the higher RPM range, roughly 6000rpm in my experience.

    On a side note, air correction jets--UNLIKE the other jets in our Webers--meter AIR as opposed to fuel. Correspondingly, going UP in size--allowing more AIR--will make for a LEANER mixture in the range affected by the air correctors. This contrasts with going up in size in the main jets or idle jets, which meter FUEL, which as a rule makes for richer mixtures.

    --Mike
     
  9. f355spider

    f355spider F1 World Champ
    Rossa Subscribed Owner

    May 29, 2001
    16,553
    USA
    Full Name:
    Hugh G. Rection
    On my 78 308, we left the air correction jets alone, and only richened the main jets....richening the air correction my have been helpful though, to finalize the tweeks.

    I am highly suspect of any 78-79 (USA spec) 308 making 220hp at the wheels without significant modifications. Did the dealer actually provide the dyno sheets, or is this just "salesman talk"? :) Only a very well running Euro 308 might make those kind of numbers.

    If your car IS a Euro 308, then I would be hesitant to do any changes, except on a chassis dyno with an exhaust analyzer to verify the base line performance, and note the changes. Which is really how you should do it with a USA car too...even though you can use my info. each car can take to the changes a little differently.
     
  10. jpl

    jpl Formula Junior

    Dec 4, 2003
    314
    Naples
    Full Name:
    JP Lavigne
    I have gone through all of the cars paperwork and I don't see any major changes and it is an early US 78 car. It was the mechanic who told me the numbers and said it was a little quicker than the rest of the CDN 308's he has driven.

    Her first owner was a drag racer/surgeon from Arizona. Maybe he did something to her a long time ago.

    Anyways she is quick enough for me, but I will probably have them tweak it a little next year to get a little more oomph.

    Thanks for all of the comments.
     

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