Back pressure on classic fabs exhaust system ?

[Keith in Norfolk]

I have very recently fitted a Classic Fabs short manifold system with link
pipes to my 4.2 2+2
The car runs fine and sounds great but after about an hour the car starts to
loose power and miss fires badly. At first I thought it was a problem with the
ignition system but after checking the usual plugs , rotor arm , coil etc these are all fine. Funny enough the problem has happened on the same journey
twice at the same spot ( weird I know ) but once I have pulled over and waited
5 minuets the car runs fine again.
Has any one else had this problem ? and what if anything can be done to sort it out . I have been told it could be back pressure form the manifolds ? or heat
build up under the bonnet.
Any help would be most wellcome

[mgcjag]

Coil getting hot and breaking down ??

[Keith in Norfolk]

No Steve , coil is fine

[Heuer]

1. Do you have electroic ignition of any kind? If so you must fit resistor spark plugs

2. You say the coil is good but have you replaced it with another known good one? When you next have the problem check the temperature of the coil; if it is too hot to touch it is faulty.

3. What rotor arm are you usingand? If fhe answer does not include the words Distributor Doctor red them buy one immediately.

4. Have you checked the fuel tank sump filter for debris? It can get stirred up on the move and clog the intake filter.

[abowie]

I have no idea what sort of R&D went into the Classic Fabs headers but I can't imagine that they are so bad by virtue of back pressure alteration as to prevent the car from running in the way you describe.

My money is on heat causing your running problem. It's more likely ignition, and specifically the coil. I suppose that alteration in cylinder gas flow could cause the spark plug temperatures to get unusually high and affect spark, but I don't really know enough about that to comment.

I know that you think your coil is not the problem. Nevertheless I would drive the car til you get the symptom, then quickly plug in another coil know to work and drive it again. All you need do is swap the two LT wires and the HT wire onto the new coil. You can just tape it in place next to the old one for the purposes of the test. If the problem persists then it isn't the coil.

If that's not it, look next at the condenser. You can check the condenser easily by hooking up another one in series. Assuming negative earth, connect the lead to the + coil terminal and earth the condenser body (you may get a shock doing this). It will be immediately evident if the coil is the problem.

Check your timing with a timing light, or better, by static timing it with a test lamp. If your timing is out and the headers have changed the combustion chamber temperature this could affect running.

Finally, replace and properly gap the points, if you have them.

I suppose some sort of fuel vapour lock is possible but I'd consider that unlikely.

[ralphr1780]

Has your temperature gauge been reading normal as usual when this happened?
As said above, I would start checking the ignition components and investigate possible fuel starvation due to clogging or tired pump.
Keep us posted on your findings please.

[Quattrofrank]

I have classic fabs long manifold so my experience might not be helpful for you.

No issues with either heat or misfiring when hot. My issues with misfiring is until the car reaches temprature. After this the car behaves very well.

As suggested the issue is most likely to be other components then the exhaust.

[Heuer]

The Classic Fabs system uses venturi's in the silencers which means that despite the big bore pipes the engine 'see's' a 1.75" system - basically you get a faux big bore system with all the macho noise without the inevitable torque dip between 2,250 - 2,750 rpm because there is not enough speed in the exhaust gases to extract themselves. Think trying to blow up a balloon through a 2" pipe. The standard Jaguar manifolds are perfectly designed for the XK engine and the 1.75" exhaust system matches. If you want more power simply get straight through silencers (the resonators are already straight through) which will give you a 6% hike in power and more noise.

This shows the standard venturi on the left, quieter (Swiss) venturi on the right:


I can't for the life of me think why the venturi would cause any sort of misfire and whoever suggested this clearly do not understand the way the system works. My money would be on 1) rotor arm or 2) coil hence my advice to get one of these:


I had a misfire on my Elan at 4,000rpm and nobody could understand why. Fitted a DD arm and it went away.

[christopher storey]

I agree with David that coil/rotor arm are the two most likely sources of problems . It may also help to check that your engine is properly earthed, as a poor earth is also known to cause misfiring. Another check is to see that there is no significant play in the distributor shaft bush, because if play is present , the points gap when running will become very erratic because the lobes of the cam will describe irregular arcs. I'm not sure why this would be heat sensitive, though . Another thought is to check the resistance of plug lead and caps both cold and hot. This should be about 5k ohms from memory, but if the resistance is much more than this, or even worse if the caps become open circuit when hot, this could account for the problem

[cactusman]

Agree with the other posts. Coil or arm probably. A failed plug top gives a misfire but not complete engine stop. Worth checking them too thiugh. They should be around the 5000 ohm (5k) mark. Get the DD red rotor arm what ever else you do.