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carbed 308 idle

Discussion in 'Technical Q&A' started by adesalos, Nov 24, 2003.

  1. adesalos

    adesalos Karting

    Mar 19, 2003
    222
    Texas
    My car when hot idle at 1500 RPM. It seems too high. What can I do to reduce that?
     
  2. 308GTS

    308GTS Formula 3

    Dec 27, 2001
    2,220
    TN
    Turn the idle adjustment screws counterclockwise. (flat head screwdriver) You will see them on the rear and front bank carbs. There are 2 total. They are on the left carb(front carb and rear carb) if you are looking at the engine from the rear of the car. Turn them in equal portions so the flow is still balanced. It is best to use a synchrometer for this but if you make half a turn on the front bank then make half a turn on the rear. Set your idle to 1000-1100 rpm and your set.
     
  3. 308GTS

    308GTS Formula 3

    Dec 27, 2001
    2,220
    TN
    Here is a pic of a carb from a fellow Chatter. This carb doesn't have the adjustment screw as it sits on the right hand side not the left but if you look at it you can see a hole on the right side of the carb that looks like a screw would go there. The carbs on the left side, front and back will have a screw and this is the adjustment screw you are after. HTHs.
     
  4. John Miles

    John Miles Rookie

    Nov 4, 2003
    19
    It is REALLY hard to see the idle adjustment screw for the front two carbs with the air box on the car. I would suggest taking it off and using a synchronizer to do the job right.

    It's unlikely that you can keep the airflow balanced optimally between the two banks just by turning the front and rear screws an equal amount. The slop in the linkage is rather asymmetrical (something that makes little sense in print but perfect sense when you see how it's put together with ball-and-socket joints that have to be kept under tension in a particular direction.)
     
  5. 308GTS

    308GTS Formula 3

    Dec 27, 2001
    2,220
    TN
    You can definatley keep them balanced if you do it evenly. I have tested it many times. If you can keep an eye on the screws and turn half a turn as an example they will be very very close. I certainly wouldn't go out and buy a synchrometer just to lower the idle. If you plan on syncing your own carbs etc. then it would be a wise purchase.
     
  6. adesalos

    adesalos Karting

    Mar 19, 2003
    222
    Texas
    308GTS,
    Why would I set my idle at 1000-1100 rpm, and not lower?
     
  7. 308GTS

    308GTS Formula 3

    Dec 27, 2001
    2,220
    TN
    Set to 500 if you like but it isn't an automatic. I don't care, your car.
     
  8. f355spider

    f355spider F1 World Champ
    Rossa Subscribed Owner

    May 29, 2001
    16,562
    USA
    Full Name:
    Hugh G. Rection
    Adesalos,
    The factory spec is 1000 rpm + or - 100 rpm...there is probably a reason for this....lower and the car will probably shudder and possibly die if you turn on the AC while idling, as I doubt there is any sort of "enrichment" to the engine when the AC kicks in.... I know my car did not idle smoothly much below 900 on my 78 308 GTS, it was always adjusted right at 1000.

    Dave
     
  9. pete04222

    pete04222 Formula Junior

    Nov 1, 2003
    613
    Maine, USA
    Full Name:
    Peter Cyr
    Sounds to me like someone has disconnected the low speed points. Not really a problem if you can live with the high idle. Without the low speed points you will not be able to get the idle below 1000.
     
  10. Steve King

    Steve King F1 Rookie

    Feb 15, 2001
    4,353
    NY
    Pete the R2 points are not the problem. You should be able to get the idle down to 500-600rpm on the single point set up. The problem is in the carb idle screw and air bal.adjustment. Just need to play with them and make sure the linkage is lub'ed and not binding. Also the factory spec is 1000rpm at idle. With single point set up need to time at 7 deg. Also I reset my idle to 1200rpm in the summer becayse the AC compressor takes about 100-150rpm of drag on the motor at idle. Not a big deal only a tweek on the 2 idle adjustment screws. Once the bal. is set and the A/F mixture are set the idle adjustment isn't a problem.
     
  11. pma1010

    pma1010 F1 Rookie
    Silver Subscribed

    Jul 21, 2002
    2,556
    Chicago
    Full Name:
    Philip
    I run around 12 degrees of advance at idle. Once warm it will idle at anywhere from 500 rpm (even with P6 cams) on up depending on where I set the throttle plates. More advance will certainly speed up the idle, but you can stop it down as others have suggested by small and consistent changes to the throttle plates.
     
  12. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
    2,279
    Roseburg, OR
    Full Name:
    Hans E. Hansen
    My car originally had the idle screws backed all the way out. Idled at about 600rpm. The odd thing is that it ran smoothly that way, even w/ the A/C on. However, it did carbon up badly.

    I merely set the screws to where they just made contact. Then I turned them in evenly until the idle was just right. Probably not sync'd 100% correct that way, but the engine 'behaves' well!
     

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