MkII 3.4 drops idle and eventually stalls

I’ve become the custodian of my late father’s MkII. It hadn’t been getting much use - mostly a yearly drive for it’s MOT test. I’d taken it the last 3 years. The car starts fine, idles around 900rpm, and, historically, I’d drive the 5 miles to the MOT where I’d switch off. Then the mechanic would fire her up and perform the MOT checks. The car would idle throughout the 45 minutes or so. However, it would eventually stall and refuse to restart. It could take a good 10 or 15 mins for it to restart. Battery is good. The car drives fine but I wouldn’t want to leave it idling at traffic lights etc. Longest drive on open road has been about 30 mins.

I really need to get this issue fixed, preferably before next MOT. So, I started the car for the first time in about a year (fuel is at least one year old, maybe two - it could be 95RON or higher). It took a while to start, cranked fine but wouldn’t fired although tried to. After 10 mins it caught and ran fine. I let it run a bit and switched off.

A week later, it started first push of the button, 900rpm idle but after 4 or 5 mins, it dropped idle to around 400 rpm and slightly rough idle there. Red ignition light on the dash glows. I can open throttle and revs climb as expected and ignition light stops glowing. I switched off.

I repeated the above a few days later. and same thing happened. Starts fine, idles around 900rpm then a definite drop to about 400-500 rpm and slightly rough idle.

I’ve had a few suggestions made by mechanics. MOT station felt it was a fueling issue - Might be running rich.

I can let the car idle at 400-500 rpm and wait the 40 mins or so until it cuts out (or take it for a drive and see if it cuts out then - but then I’m stranded) and feel if the coil had become hot - the coil could be the problem.

The fuel? I’ve been told I might need to raise the octane by one person and lower the octane by adding kerosene by another person. I’ve also been told the fuel deteriorates quickly - 6 months or so, so I should probably pump out the fuel and replace with a small amount of new fuel until this issue is fixed.

I’d appreciate if anyone could help me out with this issue. Thanks in advance.

Looks like after 4-5 min the enrichment device ( hisser, choke ) switches of.
At that moment the car is probably not hot enough to manage the leaner mixure.
Lots of people here install a switch to keep it on longer.
And 400/500 rpm at idle is somewhat low, most of us advice around 750rpm.
Try to adjust the carbs when engine is hot, and get some fresh fuel.

Peter Jan
3.8 S-Type/XK140/XK150 3.8

Thanks Peter. I’d been discussing this with my uncle a fortnight ago and had said it was as if an automatic choke has switched off. I’ll read up on that and I’ll research the carbs. My dad will have had books on the SU set-up.

Buy a colour tune , takes the guess work out in how rich your are or are not running

Adjust the slow running to around 700 , and don’t let it stand on tick over for 45 minutes , there is no need !!

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Thanks Ian, I agree with not letting it idle so long. I should have said to the MOT testers.
I’ll have a look for a colour tune in my dad’s garage. I’ll order one if there’s none there. I know he has a crypton syncro check balancer so I’ll go over the balance too.

It is just a basic test , but better then just a guess , SU carbs always need a tune up now and again , that’s half the fun right :blush:

I got mine off e-bay , there is always a few on there !

In any case, drain the old fuel out and get fresh fuel. The more volatile components will have evaporated off years ago. And check your spark plugs, just to see if the engine is running too lean or rich.

The other thing you need to do is give the old girl a GOOD run perhaps after you have changed the oil? Its probably a bit carboned up, there’s the old tale of the car being taken to the garage for a tune up, mech gets in, 2 gear and we’re off. Good ole Italian tune up.

check the sparkplug/s to see if they are black & sooty

its slightly possible if you live somewhere very hot, the fuel is boiling in the bowls ?

(I can idle idefinitely in 30C /90F temps)

most people put a manual over-ride switch on the starter carb, to make sure they turn off

I would drive it fairly hard locally, uphills etc, to see if it really runs fine, check the plugs first.

if the engine has a 90% cold idle duty cycle as its lifestyle, not good with excess fuel washing the bores

Current temp today is 12C. I’m in Scotland. It’s been tricky starting again but I appear to get it going by pulsing the throttle. I was certain it wasn’t firing on all 6 cylinders as its lumpy at idle and wants to stall. All 6 plugs are sooty though none were soaked with fuel.
I need to pump out the old fuel and get new stuff in. I know the tank is 3/4 full. How many gallons is that?

Unfortunately, 3 or possibly 4 dials are not working now and that’s just in the last couple of months. That’s the fuel gauge, oil and water temp. I suspect there’s a corroded earth somewhere - any idea where to look?

Also windscreen wipers are not working so possibly same electrical issue that affects the dials?

Thanks for the help people. It’s much appreciated.

You are looking at about 12 gallons for a full tank I think.
Check all the fuses and earths in the drop down dash.

Well, it’s taken me a while, but, I think I’ve found the problem. It’s the Auxilliary Ignition fuse on the fuse unit. It might be corroded, or the terminals just need a clean, but, it’s meant that the car was taking an eternity to start, running roughly, and the 4 centre dials were not operating. Basically some sort of ignition issue. I’d appreciate if someone could explain what wasn’t happening correctly.
I’m sure the carbs need a bit of a fettle as it may still run rich, but at least the car starts on the button now. I adjusted the idle up to about 800rpm after the choke switches off.
I’ll get it MOT’d, start using it and see how it runs after being driven a bit.
Thanks for all the help.