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Thread: XK120 ignition problems

  1. #1
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    XK120 ignition problems

    Hi there,
    i have a 1950 xk 120 ots.. I've been toying with it over the years and although running beautifully in the garage it starts missfiring after 2/6 miles. I've replaced all the ignition components, balanced carbs etc but to no avail.
    I bit the bullet and brought it to the recognised expert locally,who after doing everything I did, has concluded that the coil is not getting enough voltage. He has suggested running a wire directly to the coil from the positive on the battery.
    I have a kenlow fan fitted and he has said this is taking alot of the current from the system.
    he reckons the loss of voltage is due to an old wiring loom.
    I am relunctant to modify the system. He also suggested a differn't coil which only needs 10 volts.
    I'm a bit lost at this stage so would appreciate any opinions.
    Regards,
    Scott

  2. #2
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    Hello Scott.

    Firstly, there would be no harm in running a wire directly from the battery to the coil and driving for a number of miles to see if this has "cured" the problem. If it has, then you know where to look.

    If this does cure the problem, then the loss of voltage could be for lots of reasons. That's when you start fault finding.

    Of course it could be something else. The ignition coil itself could be faulty, although they invariably fail when very hot, not just after 2-6 miles. But, as above, it would be worthwhile fitting a replacement, then again driving for a number of miles.

    But it's all fairly basic stuff, and it's easy enough to measure the voltage at the coil, and measure again while the electric fans are running. Provided your generator (or alternator) is working correctly, and you have a good battery, one Kenlowe fan running should not drop system voltage enough to make the ignition coil misfire.

    But again, careful fault finding will get you there!
    2000 Jaguar X308 4.0
    1973 Jensen Interceptor Mk III 7.2
    1969 Jensen Interceptor Mk I 6.3

  3. #3
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    Hello Scott,

    the diagnoses of an 'old wiring loom' is a bit misleading. Voltage drop in the circuit will be confined to the terminations and possibly the ignition switch. Using a multimeter at each point (there should only be three or four) I would think will give an indication. Don't forget it need not necessarily be just one of the connections, it may be a small ammount at each. It's not really very difficult and may well be simpler to re terminate all the wires in the circuit?
    One point when doing voltage drop measurements is that the circuit in question must be carrying current and in the case of a coil this means the points must be closed.
    Another check, if you are in the failed state, touch each of the terminals on the ignition circuit, if one is hot that is the likely culprit, as high resistance joints get hot with running.
    Using a lower voltage coil to correct the fault is not what I would expect from an 'expert', it ignores the problem which will only deteriorate and the lower voltage coil will also fail in due course.

    Alec
    2004 2.5 X Type Estate, MK 2 3.8 (Long term restoration), 2.5 P.I. Triumph saloon, 564 Hymer motorhome.

  4. #4
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    Just to add to the good points raised by piman, if you do run a feed to the coil directly from the battery, don't leave it connected as it will not get turned off - it will also need to be removed to stop the engine - unless you stall it...

    One test that you could do is to take the feed off the ignition coil and connect it to a headlight bulb (low beam) . Then you can measure the supply voltage at the connector there under load - that should give you a good idea if there is a significant drop somewhere. It should be very close to battery voltage - less than 0.5 difference really, if it is much lower then you have to find the problem...
    2005 X-type 2.2D estate 16" wheels & michelin snow tyres for the winter season

  5. #5
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    Does the engine only mis-fire under load when hot ?

    You said you have replaced all ign components. Have you replaced
    Plugs (gapped correctly )
    Points ( " " )
    Condenser
    Cap
    Rotor arm
    Leads
    Coil
    Jaguar Keys & Remotes
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    196 London Rd
    Hadleigh
    Essex
    SS7 2PD

    07792 552423
    01702 555557
    castle.locks@btinternet.com

  6. #6
    Senior Member
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    I guess it fixed itself
    Jaguar Keys & Remotes
    Castle Locks
    196 London Rd
    Hadleigh
    Essex
    SS7 2PD

    07792 552423
    01702 555557
    castle.locks@btinternet.com

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