XK120 starting issues

I’ve never been entirely happy with the starting of my XK120. I have a standard setup, with the original H6 carbs, the engine is freshly rebuilt, so now it has compression (it had very little before). I have 2 small 12V batteries in parallel, rather than the original 2 6V in series setup. Recently I replaced the starter motor with a new WOSP geared-down item, which spins the engine over very vigorously. The timing has been checked.
My past experience with Jag motors is that they usually spring into life at the first touch of the button - this one, not! It almost feels that the engine doesn’t catch until I release the starter button, then it fires, and runs fine. It’s the same with a cold start using the thermo choke. Could this be a sign that the batteries are on their way out, perhaps? I do plan to revert to the twin 6V batteries at some stage.
Hopefully, one of you with more electrical nouse than I possess might be able to suggest a reason for these starting quirks…?

If the engine does not light up until the starter current is shut off, that suggests the voltage to the coil primary is too low to initiate spark when the starter is pulling current. This could be related to battery condition. It also could be too large a spark plug gap, and other issues.

Primary voltage to the coil can be measured with and without the starter pulling current to compare. Or one may do a spark gap test on a lead disconnected from a spark plug in those two conditions.

After checking voltage and spark strength, then details of your ignition system may be appropriate to explore. Lots of cars have ballast resistors in the circuit during running and taken out of the circuit at starting to alleviate this possibility. Next steps will depend on what your system looks like.

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Hi Chris,

Have you tried jump starting it to see if it makes a difference?


Thanks, Roger. That sounds eminently plausible to me. It does feel to me like while the starter is engaged it’s drawing too much current to fire the plugs, until it is released. It’s the same with both coils I’ve tried. I will try comparing the amperage reaching the coil with and without the starter engaged, as you suggest. There is no ballast resistor in the ignition system. I do have Lucas suppressors on the plugs, but they were an option when the car was new, so I don’t think that would be a factor.

I have a jump starter, so I will also see if that makes any difference. Thanks, Clive.

OK, I have just checked a few things: 12 volts exactly measured for each battery (2 x 12V in parallel). I assume if one is worse than the other it will rob the other one to achieve equilibrium?
9 volts measured at switch terminal of coil with ignition on - drops a bit when starter engaged. Amps are off the scale, for some reason.
Voltage rises to a maximum of just under 14 volts with the engine running.
Jump starter makes no difference to the voltage readings, either at batteries or coil. the jump starter is a bit down on charge, though.

You have to cheat………you need a toggle switch under the dash to fill the carbs with gas .
Your starving for fuel.
Once the pumps stop ticking, start the car and then kickback the switch…it makes a HUGE difference.
Back in the day you drove these cars on a regular basis, when sitting the fuel backs out.
That’s all you need aan override switch to the solenoid.
ps the twin batteries are a joke today…even in my 120 I have one 900 cca battery. It is never an issue and simple set up.
good luck

Had a similar issue with my xk140.
I had found 2 issues, once i dumped the blowback valve at the end of the pipe from the enrichment device it started at last, never seen any blowback problems since! Later i even dumped the modern hightorq starter, with the original one, and now it starts allways very easy.
Regards, Peter Jan ( xk140/xk150/3.8 s-type )

I’m sure it’s not a fueling issue. I have a manual over-ride switch for the thermo carb. I cut out the thermo choke after a couple of seconds in reasonable temperatures. The float bowls seem to fill fine with the SU electric pump.
I plan to keep the original two 6V batteries as I don’t want to get too far away from originality.

If you have the override already, check your grounds then, but the one battery cures a lot of issues.
good luck.

I would suggest shooting a 3 second shot of starting fluid into each air cleaner and see if it catches right away.

Hmm, maybe something is different from what I imagine. Battery voltage when isolated should be around 12 volts and, yes, if two batteries are in parallel they should be at the same common voltage drop. Yes, when running, a charging system may be near 13.8 volts.

The part that confuses me is your 9 volts at the coil with ignition on. On my car the coil primary voltage is the same as the battery when ignition is turned on. On a lot of cars, the ballast resistor is separate from the coil and is taken out of the circuit when starting so that the coil primary voltage is about 10 volts during starting, due to load from starter current. As the primary voltage on the coil reduces, the secondary voltage on the high voltage output is reduced and spark capability is marginalized or eliminated. If you are at 9 volts before cranking, then coil primary possibly will be even lower, maybe too low to create spark during cranking.

The ballast resistor is internal to the coil on many Lucas systems. As such, the ballast resistor cannot be taken out of the circuit during starting. And the coil can be wired backwards on the primary side which reduces a secondary voltage enhancement from the coil design.

If you have 9 volts on the coil primary voltage without starter cranking, that makes me wonder if your spark is strong enough on the secondary, high output side. It also makes me wonder if there is a ballast resistor between the coil and your ignition switch. If you have 12 volts at the battery and 9 volts at the coil, I imagine current flowing and a resistor somewhere in between.

On my XK120, I measure 12 volts at the battery with the ignition off and 12 volts at the SW terminal on the coil with the ignition on.

On your XK120, measure the voltage at the battery terminal of the solenoid. If it’s 12 volts, then disconnect the wire from the ignition switch to the SW terminal on the coil. Then run a jumper wire from the battery terminal of the solenoid to the SW terminal on the coil. See if that improves the engine start-up.

Well guys, I think I’ve finally got to the bottom of it. And, like so many of these problems, the answer was staring me in the face!
I did as Mike suggested and measured the voltage with ignition on at the battery terminal side of the solenoid - 12 volts. My coil picks up the current from one of the thermo carb terminals, the same one as the electrical feed wire attaches to. But, the feed wire was only giving 9 volts… Then I noticed, on closer inspection, that there was only about one strand of wire left unbroken where it goes into its terminal! Cut that off, measured the wire itself - near enough to 12 volts!
Took the solenoid off the thermo carb (mine is the early type with 2 knurled Bakelite knobs) tightened up the brass screws and re-attached the leads with new ring terminal and tiny star washers, and the job’s a good 'un, as they say!
One question: is there a right and wrong way round for the thermo solenoid, or does it work equally well whichever way it’s connected? You can turn the solenoid through 360 degrees on mine.
So, many thanks for all your advice - I do appreciate you taking the time to try and help with my little problem. That’s the joy of this forum!!

I was wondering about that 9 volts.
The carb solenoid is not polarity sensitive, so you can connect it either way.

Thanks, Rob!:grinning:

Nice … Drive on

Just replaced the 2 x 12V batteries in parallel with 2 x 6V Lincon classic batteries in series. Starting still has that feeling of not wanting to fire until the button is released… I’m wondering if I have a good earth between engine and chassis? Can anyone tell me where the engine earth strap is fitted and what it looks like?
Many thanks if anyone can, or has any more ideas re this small problem!

It bridges across the front engine mount on the opposite side from the steering.

Brilliant! Thanks, Rob.