I have my newly acquired 1987 XJS at the shop with a reputable technician who does the shop's Euro cars for the past 23 years. He's replace the distributor with a good used one, replace the pickup coil and vacuum advance with new units and set the timing. The manager said that they are trying to solve a problem where there is a "flat spot" between 800-1300 rpm in the ignition.

Does anyone have any ideas as to what the culprit may be on this, or any suggestions for troubleshooting?

Thanks
Bob

 

Perhaps the mechanical advance is sticking. One of the main things that has to be done inside the distributor of all the old XJSs is that the counterweights and associated parts have to be de-gunked, and lubricated with a dab of synthetic oil. A sticking mechanical advance is also one of the main culprits for overheating in these cars.

 

Yep, agree 100%.

Also the sychronising of the 2 throttle discs, very common issue. Proper setting of the discs and the crossrods is mandatory to proper V12 "get up and go".

Also the 2 rubberised bushes on the 2 brackets at the rear of each inlet manifold where the throttle shaft/s pivot are very important for synchronisation. They go missing regularly.

The setting of the TPS split voltage, and a good steady rise in volts as the throttle is opened is more important than good fuel for a V12. These TPS's are getting very old now, and internal track wear and heat have given them, a beating.

I have never heard of this "flat spot" in the actual ignition system. Fuel systems, mainly carbies, yes.

The list goes on.

 

That's a great call.
I think Grant is onto something with the throttle synch. At that low RPM you are just beginning to crack open the butterflies on the throttle bodies, and any play in that area will cause what seems like a "flat spot", or hesitation. And, YES, definitely make sure the rubber bushings are present, and in good shape at the throttle linkage pivot points. You will have endless slop in the linkage if they are not, and the TBs won't open correctly, relative to each other.

 

Plus check the vacuum capsule is working?

Greg

 

Just adding to the list, as there has been no response since the first ask??.

Could be a bad coil, or coils. There were 2 originally, and they are now OLD, like me. Replacement with a suitable low impedence electronic spec coil results in 1 only.

The carbon brush inside the cap is also know to go AWOL.

The module inside the ign amp is also a problem to some.

Engine earth strap/s not fitted correctly, VERY COMMON. They wrap around the LH engine mount, and a PITA on a good day.

 

Was this problem resolved? I'm curious about what was discovered.