Hi to alls
My Jaguar has a huge fuel consumption (20 liters for 100 km) whether in the city or on the road.
There is no fuel leak. Is it a matter of adjusting the two SU HD6 carburetors or could there be other possible causes ?
Any input is welcomed
Regards

 

That is fairly high. I would get the carbs set with an air-fuel ratio meter, then you know it's set properly.

Is your car automatic or manual transmission? The Motor magazine tested an S Type automatic in 1963 when it was new, and their overall consumption was 18.5 L/100km. I have converted my car to EFI and a modern manual transmission and on the highway on lower speed roads (100 km/h) I can achieve 9.7 L/100km.

 

for those that are more familiar, like me, with miles and U.S. gallons....that's about (according to my calculator) 11.76072785912209 miles per gallon. ...not good.

there aren't too many 100km roads in the world that climb steeply enough to make that an acceptable mileage figure, and i assume you're not moonlighting as a commercial gravel hauler with it either...,so, ya, probably running a little rich. but there should be some glaring symptoms of rich running though.

first you should probably smell the unburnt fuel. or maybe even see a slight black cloudiness in the exhaust gas itself. and if the spark plugs were to be pulled, i would think they would be very sooty, which should eventually make starting difficult.

but from what i've heard, the most likely cause would be the rube-goldbergian enrichment carb. as, i've heard, it's prone to malfunction involving its ability to turn itself on and off as it should, so maybe it's running at all times, even after the car has warmed up to normal operating temperature.

...just a guess, as usual.

 

On those carby cars around 17 to 18 miles per imp. gallon or 14 to 15 miles per US gallon for general driving. (MK1; MK2 and S type) Roughly 15L/100ks.
I have seen 22 -23 mpg (imp) (12L/100ks0 on very long runs on open roads with manual/overdrive sitting on 60 miles per hour or 100 kilometres per hour.
So, I think you do need a tune up including carbies, timing etc.
Also, as Huey says make sure the automatic enrichment device is not stuck open. Disconnect it from the otter switch at the front of the inlet manifold and see if that changes things.

By the way there is not much difference in fuel consumption between 2.4 and 3.4/3.8 cars as you are still moving the same amount of mass and drag around

 

when I bought my S type in 2004 i drove it 2,900 miles from Califfornia to Tennessee, and I got 225 miles from the twin tanks before I needed to refuel. And it was running rich.

 

My 3.4s auto gave me around 22 mpg on a long run (along the motorway from Torquay to Coventry sitting at 65 to 70 mph for about 200 miles) but around town only 15 to 17 mpg but then I have never really measured it accurately around town.

 

Thanks to all
I think I'm going to seriously look into adjusting the carburetors.
The current fuel consumption is abnormally high

 

I've seen the same numbers as Cass from my Mk2 at gentle, 110-120 km/h, cruising. Your numbers are more like I'd expect at 150 km/h on the highway or in very bad urban traffic. Values aren't always consistent. For a journey I used to make regularly, usually in a hurry, my car gave 21 mpg. I wondered what it would do if I drove more gently and the consumption was worse, 19 mpg.

Apart from the setting of the carbs, there's also wheel alignment, dampers (shock absorbers), tyres, coolant temperature to check.

 

Sorry I am a french guy.I had to convert your figures in liters and kms

• 225 miles from the twin tanks before I needed to refuel --> which makes 16.5 liters per 100km --> which is acceptable, I guess depending of the speed
• At 65 to 70 mph for about 200 miles, 22 miles per gallon --> 11 liters per 100km --> which is very low --> Cass3958 what is your tuning to reach such a feature ?
• On those carby cars around 14 to 15 miles per US gallon for general driving ---> which makes 15 to 16 liters per 100km --> which is acceptable, I guess depending of the speed
• The Motor magazine tested an S Type automatic in 1963 when it was new, and their overall consumption was 18.5 L/100km ---> which is higher than yours figures above ...

It's urgent to investigate on that consumption

 

The old Autocar and Motor fuel consumption figures include their performance testing. In general, they were worse than most owners would achieve in normal driving. How much worse is hard to say, but 10 or 20% wouldn't surprise me.

 

Gerard,
when I purchased the S type, I knew carburetted engines are not like the Xj-6 fuel injected, ECU-controlled fuel metering, So I am not complaining about the fuel consumption. In the XJ-6 I get 525 miles with a/c on, cruise control set at 75 mph, out of the two tanks before needing to refuel.

Add a on-off switch to make sure the auxiliary cold start carb is off / not running after you start the engine, as suggested by other owners.

 

22 mph is UK imperial gallons not the yank ones. As for tuning I have a 3.4s with 9:1 pistons completely rebuilt so only 3 or 4 thousand miles on the clock. BW35 3 speed auto gear box again completely rebuilt. I run an electronic distributor but not one that can be tuned. I have had the carbs professionally rebuilt at the same time as the engine but they are standard. The rear diff is a 3.54 but not touched. New wheel bearings all round which probably makes a difference. The 200 miles was all on motorways cruising at 65 to 70 mph with the occasional coffee stop thrown in so a pretty steady throttle and there are not that many hills. I started at 69 m or 226 above sea level and Jaguar Ryton Coventry is 81 m or 265 foot above sea level. I would like to think the journey back was all down hill. lol.

 

This is the region I see my 3.8 at. I've seen over 20 MPIG on my car in OD at coast ~ not high altitude.

 

An aspect of engine control that can have a big impact on fuel consumption but may go unnoticed by the driver is vacuum advance. If the vacuum capsule leaks or the vacuum line is missing, the engine loses part throttle advance with obvious consequences.

 

I had mine out yesterday, drove it a distance of about 65 miles round trip to a car show. I filled one tank and used about 5/8 of that. I am using all guesstimates because I neither refilled yet it nor knew my miles when I left, nor really where the gauge sits in relation to the actual fuel level, only the distance that the map told me and the 7-1/2 gallon capacity, anyway, I figure that although I've been tinkering on it, adjusting timing and carbs it is still a bit rich running and that my mpg is approximately 17-18 mpg. I am happy with this as the car is essentially a toy for me.
I know it wasn't much of a thing at the time, but did Jaguar themselves originally publish any ratings for mileage?

 

There is no mention of MPG in any of the original Jaguar publicity material I have. I have an Autocar magazine from 1963 which covers the 3.8 S type and it states 15 MPG but does not state Urban or cruising. It is possibly what they got thrashing the car around during their test where they recorded a top speed of 121 mph and 0 to 60 times of 10.4 secs.
I think Jaguar might not have published figures like that as I think one of Sir William Lyons many quotes was "If you can afford to buy one you can afford to run one."

 

Mine is worse. But I have 3 SUs (on a 4.2). I get about 10mpg...

 

The Motor magazine test covered 3200 miles and included a trip from London to Geneva and they averaged 15.3 mpg. The said:
"...possible to get 20 mpg when driving slowly on country roads"

 

​​​​​​
I've made a few such journeys in my Mk2 and in my Daimler. In both, fuel consumption rises fairly rapidly at speeds much above 80 mph (130 km/h). On one shorter journey in my 3.4 Mk2 entirely within the UK where the national speed limit encouraged 80 mph cruising, I drove from where I worked on Merseyside to the home of a friend in Dunblane, Scotland. The distance was just over 240 miles (386 km) and took just under 3 hours in ideal spring/summer conditions on dual carriageway and motorway. The 12 gallon (54.6 litre) tank still had some fuel left on arrival. In summary, that's a little better than 20 miles per (imp) gallon at 80 mph in a 3.4 Mk2 manual overdrive with 3.77 axle in tip-top mechanical condition.

For refueling calculations, I'd reckon on 21 mpg driving like a Christian, 16 mpg driving as our cars were intended to be driven, going down to 12 mpg in full red mist mode.

 

May sound obvious but why not just remove a couple of spark-plugs (one fed by each carb) to see if they are sooted up. If they are not then try rolling the car on level ground to feel whether the brakes are binding.
My former S-type with the 3.8 man o/d was not brilliant but I'd expect 25-28mpg in the countryside. My present Daimler 250 man o/d, with its ridiculously low diff ratio, I look for 30mpg across country (not steady dual-carriageways) but I know i need to attend to the brakes which are very slightly dragging.
Pete.