It's a lean machine

Discussion in 'Technical Q&A' started by pollockfr, Oct 26, 2005.

  1. pollockfr

    pollockfr Rookie

    Oct 25, 2005
    Warwickshire, UK.
    Full Name:
    Fergus Pollock
    Well, I'm on a roll now so here's another one. Carburettor English spec 308. As per loads of posts on the issue of airbox intake and removing the fiberglass wadding and rusty tubes I cut out all the stuffing and replaced the old filter with a K&N replacement to help breathing. Result - the car that used to idle nicely from cold with no choke now has to be coaxed into life with much accompanied popping and coughing for at least 5 minutes before it settles down to a consistent 900 RPM.Then on the road for the next 5 minutes it'll pop back on the overrun till normal service is resumed and it starts to get to temperature.Question. Am I correct in thinking its only the idle mixture thats gone lean or could I be looking for new jets as well? I did the colourtune/balance tweak at the time and the mixture seemed OK when hot.I know a rolling road session is the way to go but have any others out there had a similar experience?
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  3. Ken

    Ken F1 World Champ

    Oct 19, 2001
    Arlington Heights IL
    Full Name:
    Sounds like a "if it ain't broke don't fix it" thing. Yes, you likely made it run way too lean.

  4. TomDial

    TomDial Rookie

    Aug 6, 2004
    Full Name:
    Seems as someone has made the basic adjustmeants of the carbs with a choked air filter. Normaly idle shouldnt be way out just by removing the air filter. Some adjustments cant be made with the filter on the carbs.

    Maybe find a carb specialist. If he realy is one, it doesnt matter on which car the carbs are.
  5. CliffBeer

    CliffBeer Formula 3

    Apr 3, 2005
    Seattle, Washington
    Full Name:
    If you have opened up the airbox then likely you're flowing more air and affecting the A/F ratio upwards (lean condition). You likely find this most apparent at idle and cold start up as those are the points in time when you really need a richer mixture. You could try going up a size for both the idle and main jet and see how she runs. Incidentaly, there are some systems available online for less than $500 that provide running A/F mixture numbers so you can actually see what ratio you're running at different rpm and throttle openings.
  6. thecarreaper

    thecarreaper F1 World Champ
    Silver Subscribed

    Sep 30, 2003
    Full Name:
    leave the 55 idle jets , 60 's are usually too rich. try 135 mains and 200 air correctors. carb cars will always cough and spit when cold. NEVER MAKE ANY ADJUSTMENTS ON A COLD CARB ENGINE! let the car warm up all the way before making decisions to change anything. with the 135 main jets run NGKBP6ES and check your plugs ( 500 miles or so ). if they are now rich you can always go to the hotter NGKBP5ES .

    i have had 3 308's and i have been running 55 idle,135 main and either a 200 or 220 air corrector. i have the open air cleaners from Pierce also.
    best. Michael

    edit the points ignition are the biggest source of popping ,spitting and poor performance on these cars.
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  8. snj5

    snj5 F1 World Champ

    Feb 22, 2003
    San Antonio
    Full Name:
    Russ Turner
    Agree with all before - your car is breathing better so will need a little more gas - these cars seem to like to run a little rich anyway. Hard to go wrong with 55 idles 135 mains if everything else is tuned up.
    good luck!
  9. pollockfr

    pollockfr Rookie

    Oct 25, 2005
    Warwickshire, UK.
    Full Name:
    Fergus Pollock
    Interesting replies chaps thankyou.I should add the car has electric ignition and when warm shows no problems. When warm it idles fine and pulls cleanly all the way up the rev range and to all intents and purposes it feels no different then before (although its noisier). The popping is at cold idle/cold driving condition and evident till some heat gets to the engine. Pulling the choke helps slightly. So instead of driving straight off you have to let the car warm for 5 minutes before taking it out.And drive with the choke pulled for a couple of miles.After that it seems OK.
  10. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
    North shore, MA
    Full Name:
    THE Birdman
    For what it's worth, my 1977 308 with "gutted" airbox and Ansa exhaust decidedly prefers .60 idles and 140 mains. It runs considerably better with .60 idles than .55 and marginally better with 140 mains than 135s.

    I do not know the differences in the Euro carb cars between different years, but the 76/77 cars in the U.S. had very different cams than the 78-80 cars so the jetting requirements are different. The earlier cars had lumpier cams and seem to require slightly bigger jets.


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