Jaguar Repair: e-type rough idle, relative novice, rough note

Question
Hi: I have a 1969 e-type 4.2L with twin strombergs. The car runs great for about 10 miles but then starts to idle roughly. I've replaced plugs and installed electronic ignition but the problem persist. I'm a relative novice so any advice would be appreciated. Thanks

Answer
Hi Chris,
Your symptom narrows the problem down to only a thousand possible causes. The only way to find the problem is by testing. And the testing must be done when the car is in the failed mode meaning idling rough.

First you must warm the engine up to operating temperature and run a compression test. (all plugs out and throttle open) You should see from 130 to 180 PSI on all cylinders and little differences between cylinders.
Then check and set the ignition timing.
You then need to test fuel pressure at the time of rough idle. If it will run fast when it is in the rough idle mode you can skip the fuel pressure test.
When it is idling rough you need to unscrew the top dampener of the carburetor and using a slight side load of the pin, lift the piston of the front carburetor about 1/8 inch up while it is idling rough and note the change. If the engine speeds up and stays fast the other carburetor is too rich for some reason. If the engine dies, then the other carburetor is too lean. The engine should slow down and sound like it is running on just three cylinders. Do the other carb the same way.
When it is running rough note if there is any black smoke exiting the exhaust when you rev it up a little from idle.
Let me know the results of all the tests.
Howard