Still haveing trouble with my 1986 Jaguar XJS V12, still idles rough, backfires through the intake, takes at least 45 minutes to go onto a steady idle, but will still backfire when in early acceleration.

 

You haven't given us much info - like what has been done to the car, did this just happen or was it preceded by some work.

But my first guess would be ignition timing, from what you have described. There are other things it could be but start with the simple stuff.

 

Backfiring in the intake can be ignition timing, misfire or very lean mixture. If it revs up easily, but idles rough and backfires I would suspect your firing order is off.

 

Here is my original post, thanks for your help
---Quote (Originally by klipperman)---
Hi i have a Jaguar XJS V12, it struggles to idle. We have cleaned and flushed the fuel system, new filters etc. Nice clean gas runing through.
New plugs, and yet it still idles rough and seems to pop until it is warmed. Even then there is still a little popping when you pull away.
Any solutions?
---End Quote---

 

The signal from the coolant temp sensor being nearly short circuit could give those symptoms. The wiring in the engine valley is usually cooked and brittle by now. Not fully shoted though, that should give pinking at higher revs and really difficult starting in cold weather.

EDIT: Yes Warren is right, I overlooked the spitting back in my haste. Ignition timing is at least part of the problem.

 

Have you had the distributor apart and set the timing and do you have a ROM?

Check that all of plug leads go to the correct plug. Backfiring through the intake is caused by firing while the inlet valve is open. If all leads are good then check ignition timing next.

 

A nice classic fault is to shift all ignition cables one slot so that the whole lot is 30 degs early. But if you have timed it then ok. Another possibility is that one of the woodruff keys holding the crankshaft damper in place has disintegrated allowing the damper to slip.