Jaguar Repair: 1973 series III E type engine miss, gas piston, piston engines

Question
I have a 1973 series III E type with a problem I can’t solve.

When cold, it stars and runs great for 30 min or so. Then it will begin to develop a high speed/high load miss, which gets progressively worse. It sounds like ignition but I think that is unlikely since
·   It has a relatively new (2 year) Crane electronic ignition. Usually they work or they don’t
·   When the miss begins, pulling the choke out ( and leaving it out) will get the engine back to normal (no miss) for a while (an additional 10-15 min)
Suggestions??

Thank you
Regards
Bruce


Answer
Hi Bruce,
I agree that it most likely is not ignition because pulling a choke usually don't correct an ignition problem and it will not correct a compression problem. Since all gas piston engines only require three items to run, compression, fire and fuel with conditions on each.

Are your carburetors, 4 Strombergs with manual chokes? If so, you need to monitor fuel pressure to the carbs when you have the failure. It is no use testing when it is running correctly. This requires that you put a "T" in the fuel line close to the carbs and run a long hose out from under the hood and place the gauge under a wiper arm so you can see the fuel pressure at the time of failure. It would not hurt to put a timing light on the COIL WIRE and run the light out from under the hood also and put it under another wiper arm and tape the trigger down and tape a piece of cardboard over the light end on the windshield so the sun light will not delete the vision of the flash of the timing light. A miss in ignition can be seen in the break of a steady flash of the light. The flash of a coil wire on a V-12 looks like a steady flash but a miss will show up in a break in the steady flash.

We always said that the easiest car in the world to fix is a car that does not run. But the hardest car in the world to fix is a car the does not run occasionally. Unless you just keep throwing money at the problem you will not find the problem unless you can SEE the problem at the time of failure.

If the timing light and fuel pressure do not fail at the time the engine starts to fail, run a vacuum gauge out too and monitor manifold vacuum.

It sounds like fuel supply but run the two tests first. If it is a loss of fuel pressure at the time of failure first open the gas cap to see if a lack of venting of air to the tank is the cause. If not, check the fuel filter and last the fuel pump or fuel supply to the pump.
Let me know,
Howard