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Carb question

Discussion in '308/328' started by david bentley, May 8, 2020.

  1. david bentley

    david bentley Karting

    Jan 2, 2004
    172
    Saint Louis Mo US
    Full Name:
    David bentley
    On my 79 308, when cruising and I step on the gas hard, it stumbles...if I accelerate slowly....all is fine.
    How can I tell if the accelerator circuit is to rich or to lean?
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  3. ragtop1

    ragtop1 F1 Rookie
    Silver Subscribed

    Nov 11, 2006
    2,913
    Ontario, Canada
    Full Name:
    Larry Warren
    I always start with simple tasks. What octane gas are you using and have you changed the fuel filter ?
     
  4. david bentley

    david bentley Karting

    Jan 2, 2004
    172
    Saint Louis Mo US
    Full Name:
    David bentley
    Premium...did not notice the octane
    Changed the fuel filter a few years age....so probably 5000 miles or less ago
     
  5. Steve Magnusson

    Steve Magnusson F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    19,875
    Texas
    Full Name:
    Steve Magnusson
    First diagnostic step is to just visually confirm/deny that each accelerator pump jet is squirting when you quickly open the throttle and that the volume of fuel squirted by each seems about equal.
     
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  6. david bentley

    david bentley Karting

    Jan 2, 2004
    172
    Saint Louis Mo US
    Full Name:
    David bentley
    All squirters are working...I suspect they were size for 100% gasoline...not part alcohol
     
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  8. Steve Magnusson

    Steve Magnusson F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    19,875
    Texas
    Full Name:
    Steve Magnusson
    Unfortunately, that's the only thing in the 308 carb design that comes into operation "when cruising and I step on the gas hard". About the only other simple thing to check is that the distributor advance mechanisms are working reasonably (i.e., at a reasonable cruising RPM, the amount of advance is in the right ballpark).
     
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  9. BigTex

    BigTex Seven Time F1 World Champ
    Rossa Subscribed Owner

    Dec 6, 2002
    74,113
    Houston, Texas
    Full Name:
    Bubba
    Yiu are on the track but this will take effort.

    IF your car retains the matching, correct jets and tubes and foat level to original specs, that would be a blessing because it means no one has f&*^&^ed it up.

    Now, there IS a corrective set of jets, emulsion tubes, all that jazz, to run 10% ethanol gas, and it's out there somewhere.

    This is no for the faint hearted obviously.
    I watch that guy in the UK complain about "a whistle" at full throttle and I think "be damn gald you are not hearing an explosion at full throttle!
     
  10. BigTex

    BigTex Seven Time F1 World Champ
    Rossa Subscribed Owner

    Dec 6, 2002
    74,113
    Houston, Texas
    Full Name:
    Bubba
    Steve points out a great point on these cars:

    "Nine out of ten carb problems are ignition…." LOL
    Go up two heat ranges on your plugs and report back...
     
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  11. Streetsurfer

    Streetsurfer Formula Junior

    Dec 16, 2015
    899
    near Chicago
    Full Name:
    Ron
    Check for any broken strands in wires to points under the ignition advance plate that could be causing an open or short during spark advancement as the plate rotates.
     
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  13. david bentley

    david bentley Karting

    Jan 2, 2004
    172
    Saint Louis Mo US
    Full Name:
    David bentley
    Thanks....I will do more looking
     
  14. TonyL

    TonyL F1 Rookie
    Silver Subscribed

    Sep 27, 2007
    2,992
    UK
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    Tony
    It could be all manner of reasons but I would say the problem is in the accelerator pump circuit, check the diaphragms have not gone hard and deliver the correct fuel to each barrel, simply looking down each barrel is not enough as you cannot determine the correct volume is being delivered. Each jet must deliver the EXACTLY the same, from memory its about 23cc for the 0.35mm jet

    Rapid opening of the throttle causes a temporary leaning out or enrichment of the mixture due to the sudden change in air volume entering the engine & drop in vacuum, without the correct fuel to go with it your engine is stalling until things can catch up with you :), make sure the small non return valve is working too, any bleed by can cause fuel to be dumped back to the fuel bowl and starve the Acc pump circuit from delivery the right amount.

    Incorrectly set floats or to high fuel pump pressure can cause the same issue

    tony
     
  15. lm2504me

    lm2504me Formula Junior
    Silver Subscribed

    Aug 26, 2004
    735
    Nipomo, CA
    Full Name:
    Richard
    In order to pass my Smog check in California Jan 2020 for my 1978 308GTS ...
    Rebuilt carbs and replaced the accelerator pump diaphragms.
    Ran a small copper wire through all accelerator jet nozzles. Two were plugged and two needed cleaning.
    Idle Jets from 55 to 65
    Main Jets from 125 to 130.
    Also verified my carb throttle plates were fully opening with accelerator pedal at its stop. They were only opening about 65 degrees, I adjusted to about 85 degrees to minimize strain to throttle cable at full throttle.

    Adjusted idle mixture screws accordingly for the road.
    Runs great, accelerates way better than before and engine temperatures now always run at normal operating temps.
     

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