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I took this car for it’s MOT test and after the test, I jumped in to drive her home and it would not start. This is not a new problem, but, as the car only went to MOT tests for the last 4 years, it’s taken me until now to realise that it’s a hot start issue. I initially thought it was flooding, but, I've had cause to rethink this.
The mechanic at the garage pulled a spark plug and checked for spark. He couldn’t see any. (Although it was a very bright sunny day) Number 6 spark plug was dry after lots of cranking. Engine only cranked with no signs of even trying to fire.
I’ll attempt to replicate this, re-check condition of plugs (dry/wet) and check for spark.
Can anyone advise possible causes of non start while hot. Fuel is new 97 RON. Car starts first time on the button cold.
Thanks in advance.

Coil -- intermittent ballast resistor internal to coil's can.

Confirm you mean there's no immediate restart when hot?
But once started (from cold) stays running OK?

Thanks, I've taken the car to 4 MOTs. The last 3 MOTs they left the car running and it eventually stalled. It would not restart immediately. It possible took 20 to 30 mins of trying to start before it eventually started and ran. Revs were around 400rpm at idle. I've since raised idle to around 850rpm. I've not had it stall, but, I told the MOT testers not to leave it running this time. They did the test, let it run the minimum amount of time, but when I collected car 5 mins after test and 40 mins after initial 15 minute drive to test station, car didn't start. It started 2 hours later with no problem.
Does it run perfectly? I'd say it might get slightly lumpy the longer it runs.

Old coils, especially ones with ballast resistors inside, throw up all sorts of problems. Many years ago, on my Mk I I changed coil not knowing that there was a ballast resistor and so the correct Jag part had to be used. Got much better running but very poor starting until I swapped the old coil back when the effects were once again reversed.

My XJ6 (series 1 '72MY 4.2L) would start then die, start then die (needed a set of leads too); my Citroen DS23 just ran lumpy and no one could otherwise cure either. So after all those experiences just changed out the coil on my Mk IX without even trying to diagnose the occasional missfire
The secondary wire in any coil is very fine, they are high voltage -- 800 to 1200V devices -- so you can see the potential for ageing

Yes, my reply to you is coloured by my experiences -- but I hope you can see why
Keep us updated?

Do you have auto or manual choke?

It has the auto choke. Coil might be 20yrs old although actual mileage will be very low.
Thanks.

It has the auto choke. Coil might be 20yrs old although actual mileage will be very low.
Thanks.

My XJ6 ('72MY) only has 53k on it. The Citroen DS23 only 71k, the Mk IX 58k ('59MY).

I had the Mk I and so had manual choke. The auto-choke on a Mk II (subject to those in here with more up-to-date current knowledge) is a bi-metal strip controlled device that curls the bi-metal strip according to manifold temperature and so moving a needle. The bi-metal strip ages and so either doesn't curl enough, or ages and curls too much and keeps the choke off when starting's needed by not returning to datum.

Some versions of AED (Automatic Enrichment Device) were foul contraptions by SU dropped from later production, they were used by MG, other marques, as well as on early XJ6 (which is how I know ) There was a conversion kit to manual choke but I think I had the last one. On my Rover P5b (which AED wouldn't fully turn off) I put a fuel solenoid switch in series with its fuel supply.

Nevertheless, I'd go with the coil (£40-ish for a "genuine" Bosch equivalent new one from aftermarket suppliers) once you've tried replacing with another old one when you're in a no-start situation. I used a known good second hand one (£7.50) to check first. Then if that's a no fix then its on to the AED -- there might be versions still in stock for the Mk II from Manners or Barratts. Otherwise SU's service agents Gurlen would be able to help tell you which version and part # your Mk II had.

Best I can do for now...

Hello Straightsix,

checking for spark is the sensible first step. As you say, in strong sunlight it can be misleading and using an ignition strobe as a way of checking for spark is effective.
What is the mixture like, as if on the weak side it will make it more difficult to ignite?
Assuming good compression those are the only two factors.

Alec