Poor cold starts / rough cold idle / loss of power when cold

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  • #47399
    Glen
    Keymaster

    Hi there,

    I’ve suffered poor cold starts for years.  I had stripped and rebuilt the carbs twice.  Solved dashpot leaks.  Recalibrated temperature strips (bi-metal things) on the stove top.  Argh.

    Here’s what happens:

    Car is hard to start when ambient temp is below 15degC even if choke all the way out.

    When it does start it isn’t on all cylinders.  Or it appears to be happy enough to run on idle but when you put your foot down (even while in neutral and stationary) the engine bogs down and tries to stall.

    Then when driving off (even if it was idling happily at say 1200-1500rpm) it will bog down, misfire and basically be so bad that I can only drive on flat surfaces.  No hills or it just loses RPM and freaks out.

    Putting up with this invariably lead to fouled plugs (very black) and eventually when the car would reach 80-85deg C the car may come clear and be happy to proceed with normal power.

    Recently I discovered a few things (after battling this for 2 years)

    a) the throttles were set to idle correctly but the linkages were such that one opened much earlier than the other.  This was easy to see when closely watching.  I guess the fact I replaced the knuckles on the linkages affected them quite a lot a while back.

    b) I also had a poor batter ground condition as the terminals on my secret kill switch looked a bit dirty and were a bit loose

    Things have been much better these last few weeks but it is still awkward to get reasonable power even on choke.

    I honestly think it’s leaning out horribly when cold.  Which is odd as both choke cables are working well and I can see it dropping the jets etc.

    So I’m starting to wonder if the engine oil I’m using in the dashpots is just way too thick and preventing the dashpots from rising OR something in there is a bit bent and preventing one from rising when cold.  Maybe it comes good when warm.  It seems that it takes ages to get any heat into the carbs under cold ambient conditions.

    Given that I was getting horrible misfiring I’ve been chasing electrical issues for years.  I am on the lumenition kit and get a great spark.  All plug leads have been swapped out too.  I’m running NGK iridium plugs.  I wondered if the heat range of them was wrong.  They’re never very clean.

    Certainly having the carb linkages out wasn’t helping as one side of the engine was starved and the other was presumably way too rich.  Or both sides were rich just to keep it going as one throttle wasn’t open much.  Now that I’ve resolved that I think I can chase a few final tweaks.

    It’s such a shame to ruin such a good car by making it undrivable in cold mornings.

     

     

    Site Administrator,
    AKA Glen Johnson from Hobart Tasmania.
    Owner of :
    //www.triumphowners.com/to-car/2500-s-2-7l-pi/
    //www.triumphowners.com/to-car/spitfire-4-mk1/
    //www.triumphowners.com/to-car/stag-mk2-3/

    #47402
    Glen
    Keymaster

    I’ve recently made huge progress with this.  All of the above tweaks helped a huge amount.  But the final tweak was to fiddle very precisely with the right hand choke cable (the one that just drops the jet rather than moving the carb throttle.  I had to experiment over a few very cold morning starts.  Get it to fire, jump out and adjust that right cable, rev it in quick bursts and see how snappy it felt.  Too much cable and it would be lumpy and bog down.  Not enough and it would try to stall as soon as any load was applied.  After some trial and error I’ve reached a position where I have now had reliable cold starts (at say 5-8deg C ambient temp), relatively reliable throttle response under load, and less lumpy/bumpy running.

    I’d tried factory settings in the past but I can now say that some experimentation is best in these low temperature starts.  It’s surprising how much that second choke cable affects the car.

    Site Administrator,
    AKA Glen Johnson from Hobart Tasmania.
    Owner of :
    //www.triumphowners.com/to-car/2500-s-2-7l-pi/
    //www.triumphowners.com/to-car/spitfire-4-mk1/
    //www.triumphowners.com/to-car/stag-mk2-3/

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