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308 carburetor CO and HC settings

Discussion in 'Technical Q&A' started by Peter, Jun 11, 2019.

  1. Peter

    Peter F1 Veteran
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  2. Steve Magnusson

    Steve Magnusson F1 World Champ
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    Do you mean that you've permanently disabled/removed the air injection system, and plan to use it in that configuration?
     
  3. Steve Magnusson

    Steve Magnusson F1 World Champ
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    #3 Steve Magnusson, Jun 11, 2019
    Last edited: Jun 11, 2019
    Interestingly, on the '78-'79 US 308 models, they changed that emission label (and the procedure) to specify disabling the air injection and then adjusting the warm idle CO at the sampling tubes to be 6% +/- 1% (which seems a bit rich to me -- maybe they were intentionally trying to heat up the cats quicker). Do you have access to a copy of the 150/78 OM?
     
  4. Peter

    Peter F1 Veteran
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    The pumps have been “disconnected” for around 38 years. Everything is in place though, just in case it’s needed again (no smog testing here for years. It was costing the government too much money to run, because not enough cars were failing!).

    I’m sure I have that later manual somewhere on my hard drive. Yes, 6% sounds very rich and is maybe needed to help “burn” the residual gases in the exhaust stream (I’m guessing).
     
  5. f355spider

    f355spider F1 World Champ
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    Just curious Peter, are your carbs set up with stock jets? I know when I had my 78 308GTS dyno'd, it was found to be running extremely lean. The shop ended up opening up the main jets a little at a time until they reached a setting they were happy with. Ran great after.
     
  6. Motob

    Motob Formula 3
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    After making sure that the carbs are properly synchronized at idle and off idle (using a STE synchronometer), I set the idle mixture screws for best lean idle. Don't get too hung up on trying to achieve CO% numbers. I try for lowest HC and highest CO2 (see below) too rich or too lean you are going to have a misfire causing high HC's, they should be below 300PPM if everything is working right.
    The other good gas to check is CO2, the higher the number the better. The highest CO2 indicates the best combustion, If you can get 10-12% you are doing great.
    I usually shoot for around 4-5% CO. Be aware that your numbers are going to change once you have installed the airbox lid. I do all final tuning with the lid on and a nice long thin screw screwdriver for the mixture screws.
     
  7. Peter

    Peter F1 Veteran
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    Yes, stock jetting. I played around with jetting several years ago, did dyno runs, got decent HP & torque numbers, but fuel mileage got worse. I had concerns with prolonged running like that, that it would cause bore-washing, so I switched back.
     
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  8. Peter

    Peter F1 Veteran
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    Thanks, this is great info, especially for drawing attention to CO2 levels.

    I suppose the air box influences in respect to drawing some blow-by from the air-oil separator system? Or better to draw in some cooler air from the outside body scoop, instead of the hot ambient engine bay air?
     
  9. Motob

    Motob Formula 3
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    The air box affects airflow into the carbs. On some cars like a 275GTB/4, having the airbox on makes a huge difference in the way the car run. I just like to make all my final mixture screw tweaks with the airbox in place, as that is how the car is going to be run.
     
  10. Peter

    Peter F1 Veteran
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    Makes sense.

    So, I used the analyzer for the first time today and I was pressed for time, so didn't get a chance to test it with the airbox on. Although I have more work ahead of me, as I'll admit I didn't get around to rebuilding these carbs as I had intended to do. I was getting a lot of lean-spitting from the front bank of carbs, especially #7/8, even though I turned those mixture screws out almost one full turn out. I grabbed screen shots of the result charts and you can see how wavy the whole test session is. It was quite warm here today (low 80s F) and my radiator fans were cycling a lot, which, with the extra load on the alternator, was causing the idle speed to waver (and I could see the amounts fluctuate on the analyzer when that happened). I didn't have my stethoscope with me, so I couldn't check to see if I was getting air leaks around the throttle shaft bearings (which happened many years ago), or around the base gaskets, so that could be a problem. I grabbed a few segments out of the whole chart to show examples of the variance I was getting. The last segment, in a numerical amount read:

    CO = 2.29%
    CO2 = 9.1%
    HC = 365PPM
    O2 = 1.11%
    NOx = 54PPM
    AFR = 0.95

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  11. absostone

    absostone F1 Rookie
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    Peter how did it turn out. ?
     

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