One thing to check is that the distriibutor advance and retard is working, I had a distributor where the central shaft was a little sticky and if you reved the engine the weight flew out, advanced the timing and then the springs couldn't pull them back so it had a similar issue.

Becareful because I had an engine fire when my 1966 S-Type spat back through the starter carb, always keep a fire extinguisher handy.

 

yes, only under load or flooring the accelerator at idle.

 

I'm not going to bore you with a long story. But on one of my early drives with the car after restoration I switched from one tank to the other and the engine stumbled & backfired through the carbs the minute I tried to accelerate. Switched back to the other tank & no improvement ~ both tanks were full. This is with new pumps, pipes et al. 10 days of fiddling & checking everything & advice from many Jaguar "specialists" including checking valve timing, another carb rebuild & distributor change we discovered the issue. This sounds very like what you are experiencing.

At all times there appeared to be adequate fuel flow at the carbs & filter & no sign of air bubbles in the fuel supply. The filter bowl was always full.

Then we found a split fuel pipe on the suction side of the left fuel pump. (due to alcohol in fuel). Replaced it and all was back to normal & running smoothly.

Conclusion: SU's operate at such low fuel pressure (2 to 3lbs) that this has to make them extremely flow sensitive. That split pipe was affecting the whole fuel system even though there appeared to be plenty of fuel at the carbs & no entrained air.

Can you rig up a gravity feed from a 5 litre can to your carbs & see if this remedies the situation. You might have a fuel flow issue from the tanks/pumps/rear Y pipe etc..

BTW ~ I would not have believed it but we have never had an issue since then. So I learned something that I would never have believed. Did a heap of work for nothing that made no difference until discovery of the split nylon pipe.

Of course the PerTronix Ignitor came under suspicion which is why we tried another distributor with points & new condenser. Had nothing to do with the issue.

The culprit. Fortunately the car is now at coast away from the 10% alcohol in fuel.

 

BTW ~ In trying to move the car to where we could work on it & testing after trying things I believe I have damaged the new Borg & Beck clutch there was so much stumbling & backfiring. I detect a slight shudder now & again that definitely was not there before. We should have towed it. So I've bought a new clutch & pressure plate should it not settle down. Hindsight is always the clearest vision.

Good luck!

 

Thanks everyone. As mentioned near the top of this thread it has all happened since trying to install the lower steering column (not only the correct one but a longer incorrect one also) and this involved a lot of moving and pushing things under the carbs so I am wondering if the issues mentioned by Cass or Glyn may be at play here. There is certainly a smell of fuel and a fair bit of wetness around an below the carbs.

 

Check all that fuel pipework under the carbs that have to do with air getting into fuel supply, especially flexible hose from filter to float bowl feed Y pipe & feed from front carb to AED for leaks. If fuel is leaking out air can be drawn in. Might even have a crack in the Y pipe. SU Carbs seem very reliable but easily upset by minor issues.

Smell of fuel from the AED is common but there should be no wetness on the entire carb set up at all.

 

Is there any possibility that dirt in the carbies could do this?

 

Old bloke next door heard me playing with the car and heard it back firing. He leans over the fence and says “try this”. Its an old vacuum gauge with a bunch of analytic diagrams on it. He comes into my place and we connect the thing up to the vacuum advance and ..... nothing, no vacuum at all. I poke a tiny wire down the tube on the carb body, we get vacuum and, it appears at this stage, no backfire. Yay

 

That is good news. Small things can bugger these things around as I have learned. Maybe carbs need a complete strip & clean & fix any leaks.

Buy an Austin Mini vacuum line. They are dirt cheap & do the job very well. About 2 Pounds. If your vacuum port on the carb was fouled it is likely the vacuum pipe is too. My car had a coiled copper vacuum line to the distributor vacuum advance unit & it was fouled & both rubbers split.

Replacing your plug wires was not a waste of time if the old ones were cross firing.

 

Spoke too soon. Went out and tried it again today and backfire is still there. There's no doubt though that for a while yesterday it wasnt there.. Bugger!!!!
Glyn, I cant see any nylon or even flexible fuel pipes anywhere around the carbs. Where was the nylon pipe that you show in the photo above.
How tight should the nuts that hold the carbs to the inlet manifold be please? I can still turn them half a turn or so with only a little effort with an open end spanner.
Thanks all

 

The picture I show is at the rear fuel pumps. Nylon pipes tank to pump & pump to rear Y Pipe ~ plus thin nylon pipes forming the breather network.

There should be a front flexible hose from the AC fuel filter outlet (Fuel Filter mounted on wing) to the carb Y pipe to tolerate engine movement.

Just nip up carb to manifold nuts firmly. Don't over tighten. Sorry I can give you a torque. Never seen one published

Flexible hose: Was originally Nylon.





Fuel Filter with non standard flexible outlet pipe

 

Thanks Glyn

 

Get your man with the vacuum tester to now go to the other end of the vacuum pipe and see if there is a vacuum at that end. As I said previously my back fire through the AED appeared to be a leak on the vacuum pipe.
As for a torque reading for the carb nuts I doubt if there is one as you can only just get an open ended spanner on some of them let alone a torque wrench.As Glyn says just pinch them tight. They should have a spring washer on between the nuts and stud so pinch than as tight as you can and the washers will hold them in place.

 

Agree with Cass. Time to check the other end of the manifold & the Servo vacuum line.

Here is the rear fuel system if you need to start checking that end. 32 & 35 are the rear flexible lines both sides.

 

The starting carb feeds the fuel to fittings near the intake ports on each cylinder and pieces are joined together with hose. There might be a leak there. It's on the underside of the intake manifold, so not easy to see.

 

Thanks Jagboi - I'll have a look

 

Your car should be fitted with a B type head & manifolding.

Here is engine side of carbs that shows the AED T pipe outlet that flexible hoses are attached to if so configured that Jagboi is referencing. These off of an older Mk2

 

I have just checked the flexible hoses around the fuel pumps and seem ok. Next I have to figure out how to get at the flexible hoses under the starter carb - hard to get to.
I just wanted to describe the back fire( or blowback) a little more precisely. The car starts reasonably well particularly now that i have connected up the starter carb. If the car is simply idling and i put my foot down suddenly to the floor the revs start to rise quickly, it gives one short sharp backfire then the revs continue to rise. It only ever backfires once for each time the accelerator is pressed to the floor.

 

That would still indicate that it is briefly going lean.

What happens when you are driving the car with the engine under load & you suddenly floor the throttle?

 

I will take it onto the street later today and report back. Thanks Glyn