OK, took it out onto the street. It is running about as badly as it is possible for a car to run and still actually keep going. If I press my foot to the floor under load then it will splutter backfir, splutter again, do another back for or two then splutter.
Even under the mildest load (eg maintaining speed in third gear on a virtually flat road it is weak and splutters and has the occasional backfire.
I am pretty confident the timing is the same as before I fitted power steering so it really does point to a fuel issue. I spose the next thing is to replace all the flexible lines around the carbies that I possibly can.
How hard is it to remove the two carbies?
Obviously disconnect the throttle linkage and the flexible fueld line and the vacuum advance and the throttle springs - is there much else?

You have mentioned all the main disconnections the hardest part in my view is just getting to the nuts. Top ones not a problem. Lower inner nuts are a Bas**rd unless you have very thing arms that bend halfway along your forearm. First you cannot see the nuts so it is all by feel. You cannot get a socket on them so it is all done with an open spanner. I would advise a small stubby 1/2 spanner as the amount of room you have to wield it in is restricted and it is a 6th of a turn at a time. My advice would be to remove the lower inner nuts first whilst leaving the others tight. This way as soon as you have gone past the spring washer you will be able to spin them off with your finger tips. If you leave them to last as the rest of the carbs are loose these hard to get at ones will need turning with the spanner for longer.
As the difficult part is getting them off, buy a rebuild kit and following a good clean replace all the gaskets and valves. It is an easy job to rebuild them, no special tools needed just common sense and a workshop manual. Just take loads of photos as you dismantle so when putting them back together you have photos to refer to as to how they were.

There is something in the Gremlins hand book about other things breaking when you have not touched them whilst working on a completely different unconnected part of the car. I think we have all been there. I remember removing a radiator once on my Ford Cortina, flushed the block out, refitted a new radiator, filled it back up job done. Tried to start the car and nothing. Turned over but would not start. Several hours later I found the condenser in the points had failed. Totally unconnected with what I had been working on but had given up the ghost as I had the radiator out. Oh the joy of classic cars!

Thanks for that Rob. I have to say that i see the old jag as a hobby rather than a mode of transport. One good thing out of all this is that at least i now have working power steering.

My wife thought that once my S Type was rebuilt on a budget and on the road that that was the end of me having to work on it. That was five years ago, a little more money in the pocket so since then I have changed from steel to wire wheels. Pos to Neg with new alternator. New upgraded brake servo. New high torque starter. Re built the SU carbs. New fuel tanks. Replaced all the seat foams. New radiator. New wooden steering wheel. Re lacquered all the wood. Re built the PAS box and pump to name a few. As you said it's a hobbie and the wife knows where I am. In the garage not down the pub.

Follow Rob's sound advice. He has had his carbs off more recently than me by about a year. He has it spot on.

Your issue sounds very similar to mine. Agree while the carbs are off fit a new kit to them ~ no need to do shafts unless bad. You might have a damaged main jet diaphragm & have mentioned wetness so new gaskets etc. are a good idea.

Check your static timing just to make sure but seems the issue is fuel/air. Remember No1 cylinder is rear of engine.

Good luck!

Latest Update. I have just noticed that I get a shock from the distributor cap. If I run my hand around the outside of the cap while the engine is running I notice that I get a shock from the no 5 spark plug section of the cap. All the other parts of the cap seem to have intact insulation but the insulation of the cap seems to have broken down specifically at the no 5 lead junction.
Is it possible that this could mean that the distributor cap is compromised or is it something that doesnt really matter - where else would the spark go - its too far from the adjacent spark plug lead junctions?
Feedback welcome. If I need a new cap then if I can get your feedback soon enough I can order one today.

on the subject of ordering spare parts for the jag, there are a couple of main places which sell these things out here. One is "JagDaim" and they have a suitable cap (with acorn fittings) for $113 dollars australian (plus $9 delivery). This equates to about 65GBP or $90USD. I notice on ebay I could get the same thing for $60 AUD delivered to australia from the UK but that would take weeks under the current conditions.

If you remove the cap from the car and examine internally you can usually see tracking marks between the rotor transfer lugs for each cylinder if you are suffering crossfire from lug to lug. Usually have a dark grey appearance left by the spark jumping.

Your cap might well be cooked.

I cant see any obvious tracking, but regardless of that Glyn, if the cap is allowing spark to leak out of it (even if not to another lug) then does that mean it needs to be replaced. If there is spark leakage does that imply that the spark at the plug itself could/would be weaker than it should be?

Buy a new cap & yes with leakage elsewhere the plug is not receiving the full clout of the HT side of the coil.

Good luck!

There is also the fact that if number five is firing but jumping across to one or three, which are either side of five in the distributor, causing them to fire early or late and that might be where your backfire is coming from. The firing order is 1-5-3-6-2-4 . Not so much a badly timed engine but the fact that the wrong plug is getting juice and firing when it is not supposed to.

I have my fingers crossed that that is the issue Rob

Hi. I fitted the new distributor cap and leads and still the same backfiring issue exists. Its a bit depressing actually. However I was out listening to the engine today and it was running roughly. Decided to start removing the spark plug leads one at a time to see if I could determine which cylinder was not sparking. When i did remove a lead (any one ) I could hear sparking within the distributor itself. I would not have expected this. The rotor is still one of the old black ones. Would you expect that there would be sparking inside the distributor?

No ~ something is wrong. How worn is your rotor? You might be hearing spark jump to the cap lugs.

BTW ~ after my fuel flow issues causing your symptoms have you tried rigging up a gravity fuel feed direct to the carbs? You might still have a lazy fuel pump or pump with leaking internal valves. Switching between pumps does not necessarily fix because of the joining Y pipe between the two.

I finally admitted defeat and took the old girl to a local mechanic who is known to be pretty good with SU carbs. Turned out the problem was a fouled spark plug but carbs were worn and leaking air round worn shafts etc. so the carbs gat rebuilt and car was tuned using a dyno. The result is that it now goes like a rocket and is smooth. Love it. I am s bit ashamed that i had to take it to a mechanic and it wasnt cheap but it sure feels nice to drive now.

Ah! So it was a damn air leak causing it to go lean but at the butterfly shafts.

Delighted you have it sorted.

Glad it is now sorted and there is no shame in going to an expert. I had to do it to get my valve shims sorted and although it cost me, like you, the engine is now running beautifully and quiet so well worth the cost.

Indeed no shame. I had to go to experts for many things during my restoration. Many jobs beyond my capability or skill set. As an example I would have made a royal mess of re-doing my interior.

WOW, that must have been quite the vacuum leak to cause the back firing problems, those shafts must have been severely worn ?

He showed me the two shafts and one in particular had very noticeable wear. They do the tuning with the car on a dyno so it is pretty much at its peak performance for the current state of the engine. they measured the horse power and managed to get it up to 120 at the wheels which doesnt sound much but its comparative. To give an example a good holden 202 motor puts out about 60 at the wheels. The blokes hilux puts out 40 at the wheels. Anyway as i drove away from the mechanic’s i floored it and really got pushed back in the seat. The mechanic who did the work is no spring chicken and races cars so i have significant faith in his work.