at last i think ive got my 308 gtb tuned. | FerrariChat

at last i think ive got my 308 gtb tuned.

Discussion in '308/328' started by alan p. read, Aug 16, 2014.

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  1. alan p. read

    alan p. read Rookie

    Jun 26, 2006
    15
    essex. england
    Full Name:
    alan p. read
    when I got my 308, it was really exciting. loud, raucous, great fun. after a while I noticed a flat spot coming off idle. this was compensated by blatting the throttle. also there was spitting back from the carbs. I tried adjusting the ignition timing by rotating the dist to advancing the idle timing. this seemed to help but high engine speed misfired. so I figured the advance curve was incorrect. I had the good fortune of living near superformance when they in Hainault, Essex, so I had a great relationship with mike and ron. I still regard ron lusardi as one of my dearest friends. what he doesn't know about 206, 246 dinos an it worth knowing. I digress. after some research, I changed the idle jets from 50's to 55 as per the 246 dino. that did improve the low down performance somewhat. fast forward... black stallion ignition, loads better. after calibration, no more spitting back, on the fly ignition curve adjustment. unfortunately the bs module failed, it ceased to be, it was a ....thanks python. anyway, I didn't want to go back to points and bob weights, so I needed to think of an alternative. I modified the black stallion dist to run a trigger to fire a aldon Amethyst system. the software was much, much better than the bs system. I went back to basics. 50 idle jets etc. checked float chambers etc, etc. fast forward... 65 idle jets, 140 mains, 220 air correctors. alls good.
     
  2. Ferraripilot

    Ferraripilot F1 World Champ
    Owner Project Master

    May 10, 2006
    17,847
    Atlanta
    Full Name:
    John!
    Very glad to hear she's running well. Did the 220 air correctors work well north of 5500rpm? I found that these engines need about a 190-200 with a 140 main to really be optimal up to 7800 rpm with this modern garbage fuel, perhaps it's just a stateside fuel issue?
     
  3. alan p. read

    alan p. read Rookie

    Jun 26, 2006
    15
    essex. england
    Full Name:
    alan p. read
    I will get my 308 on a rolling road in due course to absolutely nail the jetting. so it may well need richer airs. but as I speak, it is running so nice. no hesitations, nice idle and good throttle response. I forgot to say, she's running high comp pistons. ( superformance's J E ). also, I have one of those fuel cat in the fuel feed, which I believe improves the fuel quality. I'd be interested to see if anyone has got any opinions about these fuel catalysts.
     
  4. Aircon

    Aircon Ten Time F1 World Champ
    BANNED

    Jun 23, 2003
    100,524
    Melbourne, Australia
    Full Name:
    Peter
    65 idle jets? Holy crap! I think you'll get a shock when you put it on the dyno, at least in my experience. Sure, it's covering up all the running issues, but it's going to be way too rich.
     
  5. 911308

    911308 Karting

    Jul 27, 2010
    113
    Launceston
    My observations are similar.

    The 65 idle jet provides about enough fuel through the progression circuit when all the holes are exposed to fix transition weakness but provides way too much fuel at anything other than the point of maximum transition weakness. A 57 idle seems the most satisfactory compromise. You can then chase the remaining transition issue with the main jet /emulsion tube. 190 - 200 air correctors will help bring in the main circuit earlier as does the larger 135 -140 main jet.
    Dyno tuning rarely looks at transition and idle circuit tuning at light loads but will sort the WOT mixtures. As suggested, try to dyno with some 190 air correctors to get the WOT mixtures but to also help the remaining transition weakness.
     
  6. alan p. read

    alan p. read Rookie

    Jun 26, 2006
    15
    essex. england
    Full Name:
    alan p. read
    thanks for all the input. I have gone from stock idles, step by step and my 308 is most happy with 65's. it was my gut feeling they are too rich, and I'm prepared to be wrong, but in the end its just a number. I ran 57's for a while, but still had that engine dying thing happening when gently coming off of idle. can I assume that the stock emulsion tube need changing? if so, to what? its my experience they are a bit of a black art. if anyone can give my that info, then I will re-cal to your recommendations. many thanks all.
     
  7. 2dinos

    2dinos F1 Rookie

    Jan 13, 2007
    3,023
    Did you check float height?
     
  8. 911308

    911308 Karting

    Jul 27, 2010
    113
    Launceston
    There are two different problems that can arise but need different attention.

    The first is off idle stumble and the second is transition weakness/stumble. Transition tuning is discussed here https://www.ferrarichat.com/forum/308-328/423336-dcnf-progression-tuning.html
    Off idle stumble doesn't exist with these carburetors when they are setup properly. I set these up out of the car. This means setting the throttle plates to just cover the first progression hole as they close and don't touch this setting again (no more than 1/8 turn when balancing the front left to back left air flow). Idle bypass screws should be closed and idle speed is set with your progamable ignition. More idle advance for a faster idle etc. Idle jet size and float height have very little effect on off idle stumble which is why I assumed you were talking about transition problems.

    An off idle stumble is most likely due to carburettor balance issues, too little initial ignition advance, throttle shaft air leaks etc.
    Driving on a 65 idle idle is usually like running with the choke on. Increased oil contamination, engine wear and plug fouling. If your plugs are clean and the AFRs are in the 12s then there is not much to be concerned about.
    Hope that is of some interest or help.

    Cheers
     
  9. alan p. read

    alan p. read Rookie

    Jun 26, 2006
    15
    essex. england
    Full Name:
    alan p. read
    again, thanks for the response. ok, I accept that 65's are TOO rich. I don't have my precision drill set any more, so I cant use 57's as previous recommended. so i'll try 60's again. the floats are set at 50mm. would anyone recommend 48mm? the carb balance, I am very confident, is good. Fuel pressure is 3.5 psi. which I believe to be correct. obviously I want to get this right, so I appreciate all your help. and I would like to remind everyone that the title of my thread was " I THINK ive got my 308 tuned". lol.
     

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