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308 Carb. info/suggestions on idle

Discussion in 'Technical Q&A' started by Steve King, Jan 2, 2004.

  1. Steve King

    Steve King F1 Rookie

    Feb 15, 2001
    4,366
    NY
    Just started to have some high idle problems come up. 77 308 GTB that runs very good but lately after it warms up it idles around 1500rpm . Its not the old carpet in the throttle thing nor the high idle cam stuck so I am looking for suggestions here. I did a major last Jan. (belts , valve adj,carb sync/tune) and it runns a little rich but pulls strong up to redline. Thoughts please.
     
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  3. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    21,343
    Texas
    Full Name:
    Steve Magnusson
    Another (relatively easy) thing to check is that the ignition timing at idle is where it was on the last system set-up (i.e., confirm that the distributor advance mechanisms are working OK-ish).
     
  4. 4Webers

    4Webers Formula Junior

    Nov 12, 2003
    276
    Texas
    Full Name:
    Darrell
    If it still has the twin points/microswitch system working, you should check that the microswitch is closing with the throttle at idle. The microswitch may have shifted slightly, allowing the advanced points to stay active even at idle, which would bump the idle speed several hundred rpm over the retarded points.
     
  5. Steve King

    Steve King F1 Rookie

    Feb 15, 2001
    4,366
    NY
    Dist runs up fine looking at the advance on the FW as I rev the engine. Don't know about the carb bal. yet so I'll have to check them. When I adjusted last they were all in sync. I have no points (use dual Pertronix's setup) and the plugs have about 1000 miles on them.Wires are 2 years old and fluids (oil/water) have just been changed. I need to pull the AC housing off to check linkage and acc. cable but if I do that I need to remove the engine cover. If I do that then I'll be taking it down for the winter. This is like putting my golf clubs in the basement and I hate to do that also. Oh well , Happy New Year
     
  6. yelcab

    yelcab F1 World Champ
    Consultant

    Nov 29, 2001
    11,043
    San Carlos, CA
    Full Name:
    Mitchell Le
    I had a similar problem with the carbs, idle adjusted at 900 RPM, then warms up and runs at 2000 RPM. Turns out the needle valves for one of the carbs was bad and caused flooding. Changed all four and the idle is now rock steady at 900RPM, cold or hot.
     
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  8. Steve King

    Steve King F1 Rookie

    Feb 15, 2001
    4,366
    NY
    Needle valves , interesting. Can you give me a little more insight here. Did you start running rich? Are you talking of the A/F mixture needle valves at the base of the carbs or are you inside the carb? Also this happened after the engine was warm. I know when I cold start after a few minutes it runs at my 1000 rpm idle and stays there as the temp goes up. Once I run the car for a few miles and clean out the plugs it starts to idle at 1500. Did you have the same problem? How did you determine what carb was the problem. Thanks for the info
     
  9. Dandy_Don

    Dandy_Don Karting

    Dec 8, 2003
    102
    The Woodlands TX
    Full Name:
    Don McCormick
    Steve, I have a 79 308 and had much the same problem. I solved it doing two things. 1. I adjusted the mixture screw for each carb (at base of each throat) and 2. I installed an additional throttle return spring on the shift linkage as I found that even with replacing the individual return springs on each carb that there just was not enough "return" force on the throttle shafts.

    On adjusting the mixture on each throat; get the engine to idle at 1000 or so (assume that means the engine is cold) then sequentially screw in each mixture screw until the engine starts to miss or slow, then back it out about half a turn. I found one or more of my throats to be adjusted too rich and that had contributed to my 1200-1400 high idle. Also, if during the adjustment process you find that running the screw in does not get the engine speed to change (decrease) you know that you have a cylinder that is not firing at idle. Steve Magnusson correctly diagnosed that problem on mine as I had a vacuum leak on one throat that was causing the throat not to fire at idle. I corrected the leak and was able to then adjust the mixture on each cylinder. Even after that I still had a high idle and then put in an added return spring on the linkage and this cured the problem. I now have a rock solid 1000 rpm idle hot and 900 rpm cold. Hope that some of this helps Don
     
  10. yelcab

    yelcab F1 World Champ
    Consultant

    Nov 29, 2001
    11,043
    San Carlos, CA
    Full Name:
    Mitchell Le

    What I had was one (possibly two) bad needle gas inlet valves inside the carbs. Every one of the carbs was firing as I can verify by the air flow meter. The carbs were adjusted and synched up at 900 RPM. I would drive it for a mile or two and the car would idle at 2000 RPM. At that point, it would run so hot that the headers were glowing red, so it was a big problem that had to be fixed. What I noticed was when the idle was high, there were excessive gasoline overflowing down the throats and it caused engine to idle higher. So, instead of changing one or two inlet valves, I changed all four (they were cheap) and the problem went away.

    I also had a weak return spring, so after aggressive driving and the car is stopped at a light, the engine would drop from high RPM to 1400 and slowly drop to 1000 RPM. That is OK.
     
  11. rivee

    rivee F1 Rookie

    Jan 20, 2002
    3,726
    Nowhere important, USA
    Full Name:
    John
    Steve, another thing to check is for vacuum leakage around the throttle plate shafts, and around the rest of the carbs

    Spray a little starting fluid around them when engine is running and see if the idle increases.
     
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