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Thread: Misfire mystery

Forum to discuss Austin Healey Sports Cars

  1. #21
    Yoda
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    Re: Misfire mystery

    I have a little different philosophy. I adjust using an advance light with the vernier centered (IIRC, it's 8 'clicks' total movement). You can get the timing +/- a half-degree this way, then you can tweak using the vernier for various qualities of petrol, but I've never actually bothered doing this, as the adjustment with the vernier is just a degree or two, and I don't think you'll notice any difference in 'normal' driving. Have considered setting the vernier all the way advanced, then set nominal with the advance light, and if I was forced to use a lower grade of petrol I could retard using the vernier to eliminate any pinging--I don't carry the advance light on trips--then restore to original setting when better fuel is available. Using higher octane than necessary is somewhat wasteful, but in old Healeys you can't damage anything. At higher elevations here in the colonies, sometimes you can only get 90 octane--with our R+M/2 method--but with the less dense air this doesn't usually present a problem, but if it did I could retard the timing with the vernier.

  2. #22
    Senior Member warwick-steve's Avatar
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    Re: Misfire mystery

    Thanks Bob
    All very useful
    I was surprised that changing the CB and rotor would have affected the timing albeit by a small amount.
    Steve

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    Jedi Warrior CLEAH's Avatar
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    Re: Misfire mystery

    Quote Originally Posted by warwick-steve View Post
    I was surprised that changing the CB and rotor would have affected the timing albeit by a small amount.
    Steve
    But it shouldn't. No? I've never heard of needing to re-time an engine after replacing the contact breaker. The distributor is not moved in the process.
    Hugh
    1965 BJ8
    1958 AN5


  4. #24
    Luke Skywalker Roger's Avatar
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    Re: Misfire mystery

    You should always re-time after changing the CB. The distributor may not have moved, but that's not the determining factor. What matters is where the cam opens the points, and that depends on the size of the heel, the thickness of the points, and the position on the baseplate.
    If you can guarantee they're all exactly the same as the points you're replacing, all well and good. But in my experience, t'aint so!
    Roger
    Ancient Briton
    '67 Lotus Elan S3
    '72 Lotus Europa TC
    '05 Lotus Elise

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    Jedi Warrior red57's Avatar
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    Re: Misfire mystery

    It is possible, if you replace points with a dimensionally exact duplicate and set the gap exactly the same, then, at lease in theory, your timing would remain the same.
    But, since the contact breaker is operating on a cam, the gap you set will affect how far the cam has to turn to lift the points - if you set the points with a big gap they will open earlier and if you set points with a small gap they will open later. So, you must at least check timing every time you adjust the points.

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    Jedi Warrior red57's Avatar
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    Re: Misfire mystery

    Sorry, was typing while Roger beat me to it.

  7. #27
    Yoda
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    Re: Misfire mystery

    OK, this is at the outer reaches of my electronics savvy, but the condenser affects timing in that it acts as a dead short both when making and breaking points contact, and sinks some or all of the current from the points before the coil gets charged. Condensers are fairly large electrolytic capacitors, and even electronics grade electrolytic capacitors typically have a +/- 10% rating--and condensers aren't likely to be even that 'high quality'--so changing a condenser alone could change timing a bit. If I'm full of it, fire away ...

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    Jedi Warrior CLEAH's Avatar
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    Re: Misfire mystery

    Red and Roger, thanks for the insight. As I think back, I've always timed the engine because I was doing more than just replacing the CB (doing an overall tune up). Only recently did I just change the CB on my car as a part of an ignition fault diagnosis process, but I dropped in the same brand of points and set them to the same gap, so I did not notice any degradation of the running of the engine. In fact, the engine is running beautifully. But, I think I'll check the timing!
    Hugh
    1965 BJ8
    1958 AN5


  9. #29
    Senior Member warwick-steve's Avatar
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    Re: Misfire mystery

    The CB I fitted was not identical to the one I removed although correct for the distributor so the slight difference I guess would affect the timing slightly. A real learning experience!
    Thanks Steve

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