Fun with the carburetors

Hoping to drive rather than work on my 56 140 OTS…
Have had the carbs professionally rebuilt, have also pulled the head etc… lots of stuff…. Finally back to driving. Had a nice approximately 70km drive on the weekend.all seemed well, one unexpected very short stretch of road under repair led to lots of bumping around, seemed like no big deal… on the return trip car stopped idling nicely… the usual solid 700 rpm idle turned into a 1000+ to 500 ish swing up and down. Got her home by keeping a solid foot on the gas at red lights as she sounded like she wanted to stall. Today a couple of days later, I popped the piston chambers off, opened up the float chambers, removed the jets etc, checked everything for signs of debris etc. all was clean, put it all back together and she fired up and idled like a champ. So I’m assuming something contaminated the front carb (a few small signs of gas on the air filter mount) and blew through? I’ve never seen carbs this sensitive… anything else I should look at? The old “Italian tune-up” is not such a good idea on a 65 year old engine! I’m thinking that some mud from the bottom of the gas tank got shaken into to mix on the bumpy section of the drive, it I expected to see evidence of that in the filters at the float bowls…

Same advice I gave Iain, there are other filters; tank drain plug and inside the fuel pump bottom cover.
Also check your floats; one may be partly filled with fuel and partly sunk.
I use a bent wire paper clip to lift them out.

Thanks for the tips, I did lift the floats out and they seemed to be dry inside. There was about a tablespoon or two of gas in each float chamber. What amazed me was how clean everything thing was…. As it should be just after a rebuild, but no evidence of what may have caused some sticking. I guess it doesn’t take much. With a 67 year old engine with unknown mileage an Italian tune up is not an option!

its possible you had a throttle not closing all the way, there is a few places the linkage can bind, normally a fuel deprived carb on a twin carb setup, the engine will run ery poorly

In these Jaguar engines, in my opinion, a hard driving session where the engine is placed under load, and near to max torque is the equivalant of an Italian tunep heating the engine and hopefully burning any internal carbon. No need to exceed 4500RPM in my opinion, even less RPM would do the job

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The saga continues, and a bit of searching through the forum shows me that I am far from alone!

Out for a couple of short drives yesterday (after cleaning the carbs a couple of weeks ago) the car got a bit of a lumpy idle at the end of the first drive, but fired right up when I headed out again about 5 minutes later, but then pretty much immediately got lumpy again and stalled out about 5 minutes later at a stop.

Seems that if I keep the revs up a bit I can coax it through, but if I let it drop to idle it dies. So I guess it’s time to dig deeper in the fuel system. Pumped all the gas out of the tank today (through the filler with a hand pump) and prepared to remove the tank drain / filter for inspection only to discover it has previously been rounded, and is locked in very tightly. I have read comments from others about the risk of ripping the entire assembly out of the tank, so I guess I’m dropping the tank and taking it to a specialist…

This appears to be an aluminum replacement, I kind of recall my dad replacing this about 40 years ago.

Haven’t attacked the fuel pump yet, it’s also a replacement unit “made in US” looks like it has an easily removable filter….

I am relatively new to this Forum, but have been working with and rebuilding cars since I was 15 (a bit over 60 years ago) when my Dad and I rebuilt the engine in my MGA. About a year ago the MG-TD I have been driving for 59 years this November 28th quit when I pulled up to a stop sign. This was very odd. Eventually (pulling choke made no difference) I got a reluctant re-start. I was almost where I was going so kept the revs up and made it in. Later in the day I turned the idle speed up and it got me back home; but seemed a bit off. A general check over of the usual suspects turned up nothing; so short drives do to lack of trust followed for a while. It acted up a bit a few times, and finally did so with a buddy along so it was easy for me to pull a plug wire and check the spark, which was thin and feeble looking. Once again it did finally start. Since it seemed to only occur after a bit of a drive I suspected the coil might be heating up and failing so I changed it out. All seemed well; but then I really wasn’t driving it far or often. Then one day on a short drive it went from running strong to stopping like I had turned off the ignition switch. A half mile ride on the end of a nylon strap got me home, and a quick check showed a good spark out of the coil but none at any of the ends of the plug wires. Putting in a spare rotor had it running great. Switching from new rotor to old, and the engine from running to not, several times gave me confidence in the problem having been found. For the first time in a lifetime full of cars (I’m talking old time points and condenser ignitions here; I have been involved in many high voltage electronic ignition system rotor failures) the rotor had failed. You seem to have covered the fuel supply side pretty thoroughly, could it be it’s the ignition?

have you checked along the entire fuel line to see if there is a later installed small fuel filter…often done…: if so–an inspection or replacement can be easily done. IF not–installing a clear one–between tank and fuel pump will soon tell you if the tank is sending debris–to that filter…IF not–then no need to mess with the tank…and you can remove that clear filter–or find one that has less than a 1 psi pressure drop. A problem with many common add on in line fuel filters is they can drop fuel pressure 1-2 psi…and the XK runs on only 2.5 to 3psi.

Hi Eric, thanks for your thoughts! I have been focused on fuel supply as I have already been through the ignition system and replaced everything as part of an extensive tune up…. Bought a kit from Moss, used Cap, Rotor and Points, did not use the condenser as it was too big to fit in the dizzy, read lots of comments on questionable quality of some repro parts…
But I have been wondering how come it fails to idle to the point of stalling, but when I hold the revs up it drives fine… the last two struggling drives included a 15 minute drive on the highway after the lumpy idles at in town red lights, and a @ 1/2 km steep hill climb after a stop sign stall. As long as the revs are up it seems to run fine…. My early thoughts were flooding not fuel starvation, but checking filters is easy-ish… I will have another look at the ignition side!

Hi Nick, thanks for the input, no additional fuel filters but once I get this sorted there will be!

Hard to find a very low pressure drop in line fuel filter. Of less than 1 psi drop

Hi again Dave. Your issue has been floating around in my mind off and on, and I have a few more thoughts to share.

You said “Have had the carb’s professionally rebuilt” and also “…a couple of days later, I popped the piston chambers off, opened up the float chambers, removed the jets etc, checked everything for signs of debris” in a recent post. It sounds as if you know your SU’s pretty well and are comfortable with and capable of working on them. They are pretty simple really, and once set up properly (which it sounds like they have been, both by yourself and a professional) really don’t give much trouble. I wonder if the old axiom “It’s never the SU carbs!” applies in your case as well.

Also you posted “…the last two struggling drives included a 15 minute drive on the highway after the lumpy idles at in town red lights, and a @ 1/2 km steep hill climb after a stop sign stall.”. It would seem to me that if the fuel system was able to deliver enough fuel for a sustained 50mph run on the highway, and to climb a hill besides, that it is doing its job adequately. Obviously very little fuel is required at idle.

I wish you were nearby as I have an ignition scope we could put your car on that presents a real time display of the spark profile being produced; kind of like the heart monitor in a hospital. Maybe a shop in your area that works on older cars has one? Another thought I had was to check that all is well with your ignition advance weights and springs, as well as that the cam (the part that the weights control with the lobes that open the points) and shaft it rides on is lightly lubed and free. If the advance mechanism is hanging up after running the engine up in speed you will have too much advance when the engine drops back down to idle.

Just a few things that you could consider.

Hi Eric. I know enough to be dangerous, but I generally learn from my mistakes of which there has been plenty over the years so I should be quite smart! In my late teens and 20s I together with my dad owned and worked on many mostly British cars as our hobby including a number of restorations. My 140 is a car that I encouraged my dad to buy back in 82 despite the fact that he though he already had too many cars at that time, and it’s one of the few that stuck with him. He signed it over to me about 4 years ago when he turned 90 and decided it was too much for him to drive any longer. He still gets himself around in his 98 Sovereign. Unfortunately this car is a “survivor”; despite the many cars that passed through his garage and came out much better off for it, this one never got more that the basics to keep it running. It was a little mix and match when he got it, and he was working on a full restoration of another 140 , so he tended to view this one as a parts runner.
So now that I have time in life to renew my hobbies I have taken over this car. I didn’t realize that it had not been driven at all for the last few years dad owned it, but that is apparent by the general condition, anything this old and high mileage does not do so well to sit for extended periods. My goal is to simply get it to Sunday driver condition and to that extent I have been working though all the issues that are coming up.
One of the first things I did was replace the water pump and hoses along with a flush of the block and rad as it was over heating and spewing coolant.
While I had the rad etc out I thought that was a great time to inspect the distributor as it was easy to access. It was a bit of a mess, worn, dirty, gummy, greasy…. I stripped and cleaned it, reassembled and put in new points, rotor and cap. The new condenser did not fit inside so I used the old one. The shaft does definitely show signs of wear, the dizzy should be professionally rebuilt, it was put on the “one day” list as I assumed my cleaning would get me to the odd car show without a problem.
I was also confronted with run-on which I could not cure, hence the carb rebuild . I quickly realized that the throttle shaft bushings were worn and sucking air, and the pistons in the carbs were miss-matched. So I could not get a low idle, and could not get rid of the run-on. I would have rebuilt the carbs myself but needed an expert to do the bushings, and he had the spares to make them match etc. a good experience that taught me to trust others to do good work for me!
After all this I finally started driving the car for short runs around the neighborhood to get used to it and ensure it was sorted. I quickly picked up on a strange rattling noise (I still assume) is coming from somewhere in the engine. After ruling out body panels, wheels, etc I finally pulled the head last winter, thinking maybe sticky valve or bucket. I couldn’t find anything wrong with the valve train, I did repair a crack in the head, removed an enormous amount of carbon from valves and pistons, and reset the exhaust cam which was about 5 degrees or so out of spec, and redid valve clearances. I also used an endoscope to get a peak at the timing chain guides which appeared to be in place, although the endoscope view was not great as it could not get too deeply inside the timing chain cover. Anyways, no idea what the rattle could be, but decided it was time to reassemble before this turned into a restoration!
Back on the road, and everything running well, but the noise remains. I swapped out the generator with a friends for a trial but that didn’t solve anything……

So, long story! But now I’m wondering if my idling /stalling problem is related to my noise problem! Both the noise and the stalling only happen once the engine is fully warmed up, and both go away with increased rpms, maybe the weights in the dizzy are whacking around and now also sticking? I have tested the vacuum and mechanical advance with a timing light and both seem to be providing the correct amount of advance.
I guess it’s time to pull the dizzy again and get it professionally rebuilt….

I think, as you noted, the fact that I have power on the highway and the hills tends to indicate my idling issue is not due to a lack of fuel…

Thanks for giving me a sounding board!

I would see if you can detect lateral play in the distributor shaft with your fingers,
can also be measured be a dial gauge.

I cant give you exact figures, but basically if you can feel it, you know

You could also try substitute the following parts one at a time, condensor, points, coil, dizzy cap, rotor button.

I prefer to have a known working spare part for these situations

just some ideas, I am sure you have thought of everyone of them, maybe you have parts laying around.

I have a large box full of Jaguar Ignition parts, including at least 10 whole dizzys

Have you tried a mechanic’s stethoscope or a long screwdriver in the ear, touching on different places, to identify the source of the rattle?

Condensers can be a cause of intermittent ignition failure. The condenser does not actually have to be inside the distributor. You can mount it on the outside or even near the coil. The condenser wire should be connected to the wire that connects the coil with the side terminal on the distributor, and the condenser body should be grounded.

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The thoughts AWG Tony and Rob_Reilly shared are both excellent. If I recall correctly you stayed with the original condenser. I’d definitely try a new one, possibly mounted outside as per Rob_Reilly. If you do, be sure to disconnect the one on the inside.

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Hi Dave,
Reference your annoying engine noise, my son and I had a similar annoying noise. Having repaired, rebuilt and replaced the head and after some considerable effort managed to free up and adjust the timing chain eccentric. We were dismayed to still suffer the annoying noise and couldn’t understand why a seemingly tight timing chain would make the noise. With difficulty (as you suggest) we commenced a boroscope inspection and eventually decided that one end of the tensioner spring was floating in fresh air, ie doing nothing. As an aside, this is a supposedly (PO) rebuilt engine. Being reluctant to battle with removing the head again, I decided to follow the advice of others (mainly Rob R) on this great forum, and see if I could access the timing chain without pulling the head. I am pleased to say that it is challenging but possible. I decided to fit the CAC hydraulic conversion and once reassembled and started - noise gone, bar a bit of tappet chatter! I’m not suggesting this is your issue, but a possibility. I have included a couple of pics to show before and after. We need to do more work on this engine but at least we can now use the XK 120 for its intended purpose - driving😁.

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I guess I’m going to pull the distributor out again and give it another inspection, always a possibility that some screw I did up came loose etc. along the way I will relocate the condenser to the coil mount. The fact that she runs well at speed or under load but doesn’t idle, and only once we’ll warmed up is challenging, seems to me most ignition and fuel issues get worse under load.

Hi Tony, thanks for your thoughts…. My “gut feeling” has always been that this is timing chain related. I previously dug into the engine with a plan to work my way through the head and on to the timing chain until I found some evidence of what is causing the noise, but I found so many other issues to resolve with the head that I eventually decided i just needed to put it back together before I pulled the engine and started a full restoration. I realized at that time that I could pull the front cover with the head in place if I really wanted to get back to inspecting the timing chain, and likely I will do that, and along the way undoubtedly uncover a few other things to be taken care of…. Like my aging pulley etc…

rattle mystery–crank damper? bad waterpump bearing-or shaft


The water pump is new, which is not to say it can’t be flawed but I’m hoping not! The crank shaft damper is likely original so I’m sure it needs replacing but not sure it would create the rattle I’m hearing; but if I dive into the timing chain this winter I’ll do the pulley as well….