XJ12L S1 backfiring

[nigel0658717]

I'm new to being a Jaguar owner, less than a week in to be precise, but I have a running issue with my new acquisition.

It is too many years to remember since I had a car with a manual choke, so I'm searching into my memory bank.

My XJ12 starts easily, both from cold and when hot.

It runs well on light throttle, but when I gently press further it stutters and backfires; hard acceleration is fine too. I can mitigate this to a certain extent by running with the choke out, but this doesn't seem like the ideal solution.

When the car was delivered it was very low on fuel, but I have half-filled each tank with 98ron E5 and added a bottle of RedEx lead replacement to both too.

All 4 carbs were low on oil, but have since been topped up.

Any tips or thoughts gratefully received.

Many thanks in advance.

[JimMann]

Hi Nigel, welcome to the wonderful world of Jaguar motoring, sounds like you have a nice one there, even if is causing you a bit of trouble right now. Stuttering on more than a light throttle could be many things and I am sure there will be well qualified folks coming forward with detailed advice shortly, but for me, a first check would be on the fuel filters and the fuel pumps - sounds like a fuel starvation issue more than anything,

Enjoy.

Jim M

[PaulGover]

Jim, I'm not convinced about the fuel starvation idea - Nigel says it's OK under hard acceleration - but of course old Jaguars do tend to suffer from blocked filters at various points in the fuel system, and it's relatively easy to check.

I'm no expert on the early V12 installations with carbs - mine is much later with electronics everywhere - but some suggestions:

AFAIK there's no point in adding lead substitute - the V12 engine has an aluminium head (like the XK straight six), and therefore seat inserts.

Topping up the carb dampers was a logical first step, but clearly didn't work.

Next to check is the distributor, which is notorious for having a seized advance mechanism, which I guess can be cured by a careful removal and strip-down. After that, look for air leaks, though they tend to ruin idling behaviour most. The air control in the V12 engines can be monstrously complex depending on which year's emission controls Jaguar were having to meet.

[JimMann]

Hi Paul, you are most probably correct, but fuel filters are a simple starting point.

Jim M

[nigel0658717]

Fuel filters and distributor on list to be looked at this week by engine specialist, so many thanks for advice.

Interestingly I’ve just been for a longer run, and half filled both tanks; on the return leg it ran well with no backfires. Will update if there is a relapse!

[nigel0658717]

Further to my last comment, I am delighted to report that the engine is now running well. and seems to have corralled some of it's missing ponies!

New fuel and a couple of decent runs seem to have done the trick.