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Charcoal Canister Issue

Discussion in '308/328' started by dennis_maine, Oct 26, 2023.

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  1. dennis_maine

    dennis_maine Karting

    Sep 11, 2007
    197
    ME
    Full Name:
    Dennis Haines
    #1 dennis_maine, Oct 26, 2023
    I have a US version 1979 308 GTS. I have been working on replacing hoses and tuning the carbs. The engine idles at 950 RPM with the throttles closed completely, so I thought there must be a vacuum leak. I found that the charcoal cannister is drawing vacuum through the bottom fitting, which is open to the atmosphere. Other cars I have owned have a purge valve that only allows air to be crawn through the cannister at certain times, not when the engine is idling. However, I can't see a purge valve in the parts diagram for the system, and there certainly isn't one on my car. Am I missing something?
     
  2. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,822
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    #2 Steve Magnusson, Oct 27, 2023
    No, when the engine is running, a small amount of air is drawn thru the charcoal canister to "dry out" the charcoal bed (and there is no purge valve). 950 RPM warm idle is perfectly fine for a 308 (in fact, is towards the low side of the 1000 +/- 100 RPM spec). If you have an R1 only ignition system (operating at, say, 7 deg BTDC idle timing), a consequence is that the carbs will need to be intentionally closed down to keep the warm idle speed down -- is your ignition system R1 only, and what warm idle ignition timing are you using?
     
  3. dennis_maine

    dennis_maine Karting

    Sep 11, 2007
    197
    ME
    Full Name:
    Dennis Haines
    #3 dennis_maine, Oct 27, 2023
    The car has an aftermarket breakerless electronic system, using two distributors, but the timing for both banks is driven from the rear distributor. The front distributor only, well, distributes spark. The timing is set to 7 deg BTDC.

    Since I have your attention (and thank you for your help) I have another issue I need help with. With the throttle plates completely closed, and I verified that they are closed with a bore scope, the engine idles at 950 - 100 0RPM. I don't understand how that can be so. The only thing I can think of is that there's a vacuum leak, but spraying ether at the base of the carbs does not produce any change in idle speed. I checked the float levels. I installed new idle jets and air correction jets. The air corrections are set at 3 turns out. I should add that the idle and low RPM performance is pretty rough. If you have any suggestions I would be happy to hear them.
     
  4. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,822
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    #4 Steve Magnusson, Oct 27, 2023
    Last edited: Oct 27, 2023
    Yes, that's an R1 only set-up so you will have to really close down the throttle plates to get the 1000 RPM warm idle speed.

    The throttle plates don't seal perfectly even when fully closed, and the stock DCNF throttle shaft bearings are known to be a little "leaky". Your best diagnostic here IMO would be using the STE SK Synchrometer:

    https://www.amazon.com/Carburetor-Synchrometer-Weber-Dellorto-Mukuni/dp/B00SLH03R4/ref=sr_1_1?adgrpid=1333708645207811&hvadid=83356932169306&hvbmt=be&hvdev=c&hvlocphy=66053&hvnetw=o&hvqmt=e&hvtargid=kwd-83357478229679:loc-190&hydadcr=5765_10409858&keywords=ste+sk+carburetor+synchrometer&qid=1698439512&sr=8-1&ufe=app_do:amzn1.fos.006c50ae-5d4c-4777-9bc0-4513d670b6bc

    to measure the actual mass airflow rate in each carburetor barrel at warm idle = should be something like 3.0~3.5 Kg/hr at 1000 RPM and 7 deg BTDC timing (and they should all be about the same). If you get a significantly lower mass airflow rate and/or the barrels are not very equal = that would require further adjustment/investigation. (With an R1+R2 ignition set-up at 3 deg ATDC idle timing and 1000 RPM, the mass airflow in each carb barrel is more like 4 Kg/hr so the throttle plates are more open, and the idle speed is less sensitive to small variations in the throttle plate positions at idle.)
    Some other things you might do is physically plug the large vacuum fittings on the intake manifold at cyl #4 (the brake booster) and cyl #8 (the airbox airflap stuff) and see if it makes any difference. Is the rest of your 308 stock, or have things like the air injection system, airbox airflap system, etc. been removed/modified.
     
  5. dennis_maine

    dennis_maine Karting

    Sep 11, 2007
    197
    ME
    Full Name:
    Dennis Haines
    #5 dennis_maine, Oct 28, 2023
    It has all of the stock items except the air pump and associated hardware have been removed. The vacuum system has been modified to account for this removal.

    I did pinch the break booster hose shut; it made no difference. I will buy a synchrometer and perform the suggested tests. Thanks to all.
     

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