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The car started popping ( late ignition in the exhaust itself) over a year ago, this is not just on overun but at and from idle. Goes away at speed or maybe I just don't notice it. We've covered 5000 miles like this with trips to the Mille and Le Mans and the car is running very well apart from the popping. we've upgraded to a Pertonix ignition system changing plugs, HT leads, Disti cap etc. We've stripped the carbs and tried both retarding and advancing the ignition all to no avail. Popping seems to be only on bank B and mechanic thinks that it must be an exhaust valve sticking necessitating a head removal. Any ideas ?

Removing a V12 cylinder head is not a task to be undertaken lightly. It pays to eliminate all other options first. Popping and misfiring can be caused by air leaks so check every inlet manifold gasket and vacuum line for leaks and perishing. Using a good old-fashioned Colortune unit can be useful to point you in the right direction by narrowing the search to the affected cylinders. If nothing's found, a compression test would identify where valve and/or piston ring issues are present. Mike.

Thanks Mike