Hi all,

I'm a pretty competent home mechanic but my 1974 XJ6 4.2 has beaten me

I am serious about an Engine guru coming to swindon, I will provide food/beverages and good hospitably, so if you would like to come let me know, it would be greatly appreciated

Here is what happened. I was tuning the carbs and got it running perfect. Pleased with myself, I turned off the ignition to go and get a cup of tea and when I came back it wouldn't start. I have since got it running but only on about 3 cylinders.


Here are the measures I have taken to try and get it running.

- measured all voltages in the ignition system (all good)
- checked all grounds in ignition system system (all good)
- Fitted a new electronic ignition system and coil (still only ran on approx 3 cylinders)
- cleaned the carbs/spark plugs etc
- checked ignition leads/distributor/Rotor arm etc (all good)
- fitted a replacement fuel pump
- Checked the ballast resistor (good)
- checked for vacuum leaks (cant find any)
- The spark looks good but i'm not 100% sure


The car has a manually converted AED with a manual cable which opearates two cams on top of it. I ran the engine by blocking off the AED and it was still the same

My compression tester is playing up (I think) but maybe doing a compression test is the next thing

Please comment if you think I have missed anything obvious but if you really know these engines and fancy a trip to Swindon, please come, I am free most days/eves

thanks

Rich

 

See if the three cylinders associated with one of the carbs will come alive with a squirt of carb cleaner. It it improves then you have have a stuck float valve or some crud blocking the float valve hole. Try a few gentle taps with a brass hammer or clean out the offending carb.

 

Tooo far for me, BUT.

I am assuming SU carbies?????????

If so, are that HS or HD series.

The HS has a small hose underneath supplying fuel from the bowl to the main jet, the HD does not.

Very different issues with both styles, but once we know, all good.

 

https://www.jaguarforums.com/forum/x...17/#post928571

 

I might add that a compression teat, wet and dry would be in order. Not a herculean task on these engines. A good baseline in any case.


Short of a compression test, just the feel and sound of cranking the engine before it fires. can give a careful ear a clue. Disable the ignition for this to get a good sense.
If three cylinders went "zero", the cranking will be quite irregular. If all are up, t will be decidedly regular.


Carl

 

Thanks for the replies so far guys.

They are HS8 carbs. I will be squirting some 'quick start/Carb cleaner' into the engine tomorrow to see if that gets in running on all six cylinders (even if only for a few seconds)

It sounds normal when I crank the engine so unlikely I have lost compression (but will check this)

In the meantime, I had another look at the carbs and can report the following;

- Both float chambers have fuel to the correct level
- there is fuel getting up through the venturi on each carb

However, one of the pistons is occasionally hard to lift for about the first 2mm of travel (it comes free with a slight 'clunk' when I put lifting pressure on it). I therefore swapped the pistons over (noting that the other one is fine) and it the second piston also got 'stuck' in the same manner.

I then played around with the spring loaded venturi from the underside of the carb and if I pull down the spring and then release it with a 'twang' it fully returns to its original position. however it does then stick if I then try to pull the spring/venturi down again (still using the underside spring loaded mechanism)

This has made me conclude that the needle on the piston is catching on the venturi. The needle is not bent, so the cause of the sticking is the needle housing itself.

any ideas/remedies/would this 'sticking' restrict the fuel flow on this carb??

cheers

Rich

 

It certainly would cause the problem you are having now.
Normally I'd say check the needle is not bent or out of position, but since the fault stays with the carb body not the piston that's not the problem.

Try it without the cover on. Does it still stick?

Try swapping the cover and piston over and see where the fault goes.

 

Hi Anjun

Its the same with and without the cover on and with the pistons swapped. Its defintely the needle sticking slightly in the jet.

New jet? Re-centre the needle in the Jet?

Rich

 

I had all sorts of problems with mine, replaced, Main Jets,jet tubes,floats and needle and seat in float chambers, idle and mixture screws.

Been working well for nearly 3 years now.

 

Try

How to Center the Jets on Su Carburetors: 9 Steps - wikiHow

Further discussion on
http://www.britishcarforum.com/bcf/a...p/t-68729.html

Did you keep all the bits for each carb separate? A common fault is to mix the parts up.

 

I thought the HS8 had spring loaded needles, so this bias thing was eliminated??????

The HD8 were fixed from memory??

The metal locating tab on the body itself, that the piston groove locates in, can be bent too far outward and thus bind in that piston groove, causing grief. A small tap with a hammer sorts it quick enough.

 

I agree with anjum, can't be too careful.





Your memory serves you well Grant.

 

Caveat:


I've only messed with similar carbs a few times with mixed results. But, also with needles in other carbs.


Smooth and straight are the watchwords. As well as an unnicked seat.


A good eye or even a jeweler's loupe to view the mating surfaces for smoothness.


Carefully wipe with mild emery cloth. Do not change the contour. Merely smoothen.


Roll the needles on a glass flat to detect and bend. Gotta be durn near perfectly straight. Any bend means a sticky needle.


Oh, another caveat. Absolute cleanliness. Stray bits will mess things up .


Carl

 

Update from Rich - Maybe a bit of progress - at last

Thanks again all.

I sprayed 'easy start' in engine today and still no difference.

I still had a nagging feeling that it was spark related and so forced myself to check the spark at the spark plugs for a third time. (spark from coil is fantastic)

Cylinder 1 spark plug was firing okay (enough to ignite but left a lot of carbon)
Cylinder 2 - NO SPARK AT ALL!!! It was sparking when I checked it the other times!!!! Arrrrgh Intermittent problems are always the worst

To be sure I then put cylinder 1 spark plug on cylinder 2 lead and it sparked. This told me my HT leads were good but that I definitely had at least one bad spark plug. New NGK spark plugs have now been ordered.

For reference the spark plugs which have gone bad were Bosch and have done less than 300 miles!!!

Will update again when I have fitted new plugs. Fingers crossed

Rich

 

Bad plugs? That is sure a newish anomolay!!! What can go astray? + electrode just burnt away, or close to it? I've seen that. Carbon or oil fouled ? I've seen that.


Cleaning sound plugs seems to have gone out of favor. Once, quite popular. I have a replica sand blast cleaner. I use it on the plugs for my yard machines. Works great.


No need on my Jeep Or LT1 powered Jaguar. They fire just fine.


Carl

 

Damn Right Carl - Im old skool and clean everything but I sure as hell aint cleaning those spark plugs for a tenth time.

Remember what I said in my original post? I had the car running perfectly - then I turned the ignition off, had a cup of tea, tried to restart it and nothing.

I think sherlock Homes said something like 'the facts are the facts however unlikely'

A broken spark plug!!! Its only the second time I have ever heard of it, the previous time the owner of a 1970 VW beetle sold the old bug cos he had lost the will to live. The new owner then told him one of the spark plugs wasnt working. Thankfully for him, he was past caring. I know how he felt, cos this intemittently firing spark plug has taken me 35 hours to diagnose because its such a rare occurence!

 

Good result.
You should still investigate and resolve the sticking carb though.

 

Rich's update following fitting of new spark plugs - still a work in progress!!

Hi all

- Fitted new spark plugs and car immediately ran nicely (yet rich) on all 6 cylinders
- Checked the timing and it is spot on
- adjusted idle speed and mixture in carbs and realised that even on the leanest setting it was still running rich (can tell this from experience (exhaust emissions/smell etc)

- Switched the car off and had difficultly starting again when warm

My current thoughts are as follows

- The AED with the manually converted 'cable pull' cam top is constantly and inconsistently adding fuel to the inlets whether it is sopposedly 'open' or 'closed'

- Or a hard to find vacuum leak?

To test the AED, I blocked off the fuel supply to it (and made sure there were no vauccm leaks). In response the car would start, run/rev up for a couple of seconds and then die (it did this consistenlty during 5 or 6 starts - maybe it was using up all the petrol left over in the AED float chamber

I then reconnected the AED and the car started running erraticatly (the idle quickly rose to 2000 revs). I then manually opened and closed the manual cable operated cams on top of the AED and this effected the revving of the engine (it dropped with cams on the 'AED' unit pressed down (supposedly in the fully open position??), and then went back to 2000 with the AED unit supposedly closed (cams disengaged)

Any suggestions/thoughts? Perhaps AED is kaput and keeps adding fuel when it shouldnt?

Manual choke conversion??

There is no further 'lean' adjustment possible on the carbs as the screws are fully backed off and jet tube is right up to the top. I certainly dont wont to fry my cylinder bores by running it rich

If a manual choke conversion - any suggestions which one?

Any other thoughts

cheers

Rich

 

I am with you on the AED. They were fickle at best.

I know you are in the UK, but a mob here called SU Mudel/Fabre, in Sydney NSW, are the SU agents, and extremely good and knowledgeable.

www.sumidel.com/

They supplied me a manual choke "kit" for my Daimler S2 with HS8 SU's, about 25 years ago, and it was a simple thing to fit.