I have a 1968 S Type 3.4 with 9:1 ratio pistons. Everything else is standard, Valves, cams, head, carbs which are twin HS6 SUs. These have recently been rebuilt with all new gaskets. I have changed to electronic distributor and done away with the points but that is all. Last year I had all the cams followers professionally shimmed to the correct gaps and the Garage, a Jaguar Indy, set the timing correctly for me. I am in the UK and using super unleaded fuel which is 97 octane.

My problem is over run.

Every time I switch the engine off on the key instead of just cutting out it gurgles on for a couple of seconds then burps and shudders to a stop.

Today I pulled all the carbs to bits again. I have a pair of colortune plugs which I set in 2 and 5 cylinders and I adjusted the carbs so the plugs were burning blue. The butterflies are both correctly closing together and I reset the idle screws to run at 600 rpm as mine is an auto box. I checked the oil level in the piston pots and topped them up so I have good damping.

And yet it still runs on.

The only things I have noticed is the throttle springs are not fully returning the throttles so the idle does creep up to 800 maybe 1000 rpm and if I slip my foot under the throttle and pull back the revs drop to 600 again. New throttle springs have been ordered which might be stronger than the original ones still on the car. Also the front carb hisses louder than the rear carb but cover either carb mouth with your hand and the engine tries to stop.

So please help me out here. What could be causing the running on of the engine when I switch the ignition off?

 

Rob. My car with 8:1 pistons but of course shaved head runs on if the throttle does not snap shut completely ~ 600RPM. Of course the idle circuit screws also need to be adjusted correctly. SU Carbs do not idle on slightly opened butterfly's as conventional carbs do. They idle on the idle circuit.

I have had to do endless work on the throttle linkage to get the idle to settle immediately and I'm using a very light helper spring on top of the 2 standard springs. Hoping things will loosen up with use & I can remove it.

If the butterflies don't snap shut completely there is fuel at the jet & these things will run on. They must settle to proper low idle to obviate this. Any form of high idle causes my car to run on before the engine stops. All tuning is perfect & carbs as new with complete new kits.

Presume your needles are properly centered in the jets and no air leaks at the butterfly shafts.

My engine is newly rebuilt so no incandescent carbon anywhere & you can't have done much mileage since rebuild & good UK additised petrol.

 

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Also, make sure the butterfly valves are seated correctly, if you have the carbs off, close the valves shut and hold the carb at a bright light.
There should be an even aura around the entire perimeter of the valve.
The darker the better.

What does rebuilt mean, these carbs suffer from people not adjusting the springs correctly ?
What ends up happening is the butterfly valves take the load of the strong accelerator return spring.
This in turn wears the shafts out and the valves can't seat properly.

There is also a seal on them, but a new one is made by Burlen to deal with ethanol.
The old cork ones don't survive with ethanol.
If none of this was taken car of, then you will have vacuum leaks around the shafts and cause you problem.

I have insulators on mine to keep the heat sink problem away.

Check you float valves too, what are you running ?
If you're running Grose jets, rid rid of them.

 

Thanks Guys.
When the carbs were rebuilt everything was replaced so all the shaft seals were done and the old cork seals got rid of.
Not sure what make the float valve jets are they came with the SU rebuild kit.
Butterflies were both set with the carbs off the car and then checked again on the car so I know they seat properly and they were matched and adjusted by releasing the pivot bolts so I know they are ok.
Needle jets were again centred correctly. If these aren't centred the pistons will not rise and fall smoothly, mine do.
When tuning the carbs the idle jets were turned up to about 1000 rpm, about three turns. The mixture screws were adjusted to get the right colour on the colortune and then the the idle screws were both turned down to about two turns to get the car to idle at 600.
I have possibly done 5000 miles since the engine was rebuilt but lots of small journeys and lots of stopping and starting.

I have new throttle springs arriving Thursday or Friday, which I will fit and hopefully these will be stronger than the 50 year old ones I have on the car and will slam the damn butterflies shut. I have actually order three, one C13355 which is the long one and two C14366 springs which is the shorter one under the front carb. I will see if these help if not I might add the extra C14366 somehow to backup the two main springs.

One thing I did notice yesterday when I was checking the damping of the SU pistons, if you put your finger in the mouth of the carb and lift the piston should it be on the oil damper straight away or should there be a small lift with no damping. On both of my pistons I can lift them 3 to 5 mm with no pressure from the oil damping. You can flick them up and they snap back down. So at what point should the oil dampening take effect?

I should add that the engine is running fine, pulls well, no stutters or hiccups, clear exhaust but the plugs were a little dark and sooty but hopefully the retune should sort that.

 

The damping action is correct, it designed that way to act like an accelerator pump on a North American carburetor.
That slight movement you get from the piston before the dampening takes effect is normal, it gives a slight shot of fuel on acceleration or from a stop.

It's quite possible that retuning will solve your problem if it's running a little rich.

Does the colour tune require that the rpm be set at 1000 rpm, the faster the engine is running the more difficult it is to set the mixture, at least when doing it from experience and listening to the exhaust note.
If all the plugs are of equal colour, lean out the carbs by 1/8 of a turn _ take it for at least a 20 minute drive and check again.
I've always set the mixture at idle, but this is by ear, it would be impossible to tell if the mixture is correct at 1000 rpm. _ maybe the colour tune is different ?

If the compression is even with in 2 or 3 pounds of each other, then both carbs should be very close as far as mixture is concerned.
You can check this with a depth gauge when the dash pots are removed _ getting them equal will give a starting point to set the mixture.

The new needle valves from Burlen will have Viton tips, and if they came with the kits, it's very unlikely they are Grose jets, since Burlen does not make those.
The ones prior to that were solid, probably brass, bronze or nickel.

The above all assumes that the carbs are correctly balanced.
That has to be done before setting the mixture and then checked again after one is satisfied with the colour of the plugs.

 

For years on my Mark 2 I have used a Unisyn to balance my carbs. However it is hard to get an accurate balance with the little ball jumping up and down.
I just bought and used a Synchrometer by STE and the balancing is so much easier since its readings are very steady. The only difficulty is that on a Mark 2 you need to make an adapter that will allow the meter to fit between the carb flanges and the inner wall of the engine compartment.

 

Not sure if the colortune requires 1000 rpm but the blue book states you open the idle to bring the revs up prior to fiddling with the jet screws then return the idles after. I will have to go back in and check again once the new springs arrive.
Can anybody give a definitive answer as to why the engine would run on? Is it over fueling, under fueling, over heating, bad seals and too much air, open butterflies, Ethanol fuel, cheap fuel ie 95 ron or all of the above and many more not mentioned?

 

Rob ~ I can confirm my carbs allow small lift of the piston before damping commences. As Jeff says it acts like an accelerator pump ~ brief mixture enrichment as you accelerate. If the pistons were rising too far or too fast you would suffer engine stumble/flat spot at pull away.(too lean)

I'll put money on the fact that your throttle butterflies are not closing quickly & properly due to Jaguar's crappy throttle linkages and the very light long & short throttle springs.
To prove it to yourself. Stop the car. Put it in Park. Open the bonnet & give the linkage a shove by hand to take idle down to approx 600RPM. Then turn off with key. I'll bet it wont run on.

The run-on is caused generally with SU's by false air. Butterflies not properly closed, air leak at throttle spindles or carb flanges etc. (incandecent carbon, timing etc can aggravate but you know what you are doing). Remember there is always fuel available at the main jet. The needle does not close it. Old air cooled VW's were chronic for doing this until they fitted an electric fuel shut off valve to their carbs.

You know what JCS has to say about Jag throttle linkages on the Register. He is correct!

 

Thanks Glyn. It only does it on hot days when the engine is hot. Today it is pouring with rain here so it will have to wait until later in the week.
I think you are right though so I am hoping the new throttle springs wil help but they will not be here until Friday possibly.

 

Hot days makes sense.

 

My 66 3.8 hs a whistle like device that runs from the white ignition wire and opens when ignition is off. Apparently it creates a huge vacuum leak to kill the run-on in our hot desert climate? No IDEA if its original or an add-on. My 3.4 didn't have it

 

Any chance of a makers name or a photo steve. I do not think it is original but might have been a local extra if the car has always been in a hot climate.

 

I'll get that info tonight. I seem to recall someone on a forum identified it fifteen years ago and I haven't looked at it since LOL

 

Maybe try a set of colder plugs. Run on can only happen if something is hot enough to keep the fuel igniting.

 

One would think so. Suck, blow, bang. The number of times I have said to someone having problems starting a car that all you need is air, fuel, combustion and a spark. I have taken away the spark by switching the engine off so my problem must lie with the air, fuel or combustion. 9:1 ratio pistons might not be helping on the compression side. A throttle that is not closing properly might be a cause for the air and fuel.
The new throttle springs arrived today and before fitting them I gave the a comparison test with the ld ones. First of all the are the same length so the old ones has not stretched. Next I hang a four pint carton of mild off the bottom of the springs and measured the lengths. All pretty similar within a couple of mm of each other which was disappointing. A part from the new ones being shiney there does not appear to be a lot of difference in them.
Fitted the new ones to the car but I decided as I had bought two to double up on the shorter spring C14366. Makes it a lot stronger for closing the throttle. Started the engine and the idle after warm up with the new springs in place was below 500 rpm having been above 600 before. The double spring is definitely pulling the throttle closed tighter. Adjusted the idle back to 600 and all appears well but it is not hot enough today to get it up to a real good temperature to check for run on.
When I was fiddling with the springs and trying to turn the butterflies by hand to get the springs on I did notice that they were very stiff but this is due to the kickdown cable and mechanism being stiff. This might be a reason why the throttles are not snapping shut so I might look into a new kickdown cable.

 

Just looked at the price of a new kickdown cable from SNG £114.77
I thinks me might take off the old one and oil it up.

 

Cass one point if it applies. If you have an electric fan in the system and it is wired w/o a relay it can provide enough energy to run the ignition for a bit until it spins down. Have seen this many times.

 

No electric fan fitted so not that

 

Rob. The automatic accelerator linkage is even more complex to get loose & clean in it's operation. But do the key switch off test at a manually closed throttle at 600 RPM. Getting that right kills all my engine's run on issues in all conditions. If that works at least you are not chasing ghosts. If that does not work we can discuss matters further. I'm pretty certain that your throttle butterflies are not closing properly if the revs can creep up to 1000.

Auto ignition/dieseling can certainly take place in a hot gasoline/petrol engine with no spark but only fuel & air available. High compression can aggravate this but your compression is not high by modern standards & UK fuel is excellent.

I'm not going to get into a long discussion on octane which would help this as I don't think it serves any purpose here.

The higher the octane the lower the auto ignition propensity or potential of knock.