V12 uneven acceleration

[marcus]

Can anyone shed light on this small issue I have just started to experience please?

Under relatively hard acceleration, the engine will jerk and hesitate, with the occasional popping noise from the exhaust. It'll still accelerate, but quite hesitantly.

Under gentle or more normal acceleration the engine performs just fine.

About 8/9 months ago I seem to remember I had the same issue, so then I changed the following:

Swapped the Opus for a Lumenition ignition system, incl new coil and gubbins inside the distributor.

New plugs, leads, rebuilt fuel pump, clean tank, new fuel and compressed air blown thorough the fuel feed hose from tank to carbs.Balanced airflow and mixture and reset timing.

Everything was just fine until the heat of this afternoon's very short spin around the block.

Prior to going out on the road, I had been tinkering with the carbs in the driveway with the engine running and it was getting very hot in the engine bay with the lack of airflow. Don't know if this is part of the problem.
In my relative inexperience, I'd say it was electrics rather than fuel
Any advice appreciated.

[mgcjag]

Hi Marcus......could be a faulty fuel pump...you have 2 on the S3...a friend had similar problems.....ran fine on one pump but at higher revs the 2nd pump was not operating properly and supplying enough fuel......

[malcolm]

Although electrics sound a natural choice for areas to investigate, I must say my S2 4.2 did exactly the same thing when I got the mixture wrong after fiddling. Started again from factory type settings as per threads on here, and all is well now.

[marcus]

OK Steve. Fuel pump completely rebuilt with new points and diaphragms etc some 6 months ago, so I've ruled this out.

Malcolm. A possibility I guess, as I have been fiddling with the mixtures. Will revisit this area. Thanks.

[Yeldaw]

I had a similar problem and traced it to the centrifugal advance in the distributor sticking, oiled the foam in the centre of the distributor under the rotor arm, checked it sprang back when turned counter clockwise and it's running fine now. Not sure if this works for the lumenition ignition system, see link below http://www.jag-lovers.org/books/xj-s/05-Lucas.html under section centrifugal advance seizing

[marcus]

I checked the weights some 8/9 months ago and freed them up so I don't think this is where the issue is. But appreciate your thoughts anyway. Thanks.
A drive out in the motor yesterday was without any problems, so I have an intermittent problem.