CJ Hinckley wrote about the idle speed on the 66 Mk II 3.4
Somewhere in the dim dark past I recall a similar problem being discussed.
The cure in that case was one of the machined holes underneath the
manifold - an inlet for one or other of the vacuum accessories that was not
required on that particular car. Bear in mind that these manifolds often
were made for several different uses, and modified to suit. After some work
it turned out that one of the ‘bungs’ had been removed fro cleaning and
inadvertantly left off with a resultant large vacuum leak into the manifold.
It was very difficult to find, but was the cause of exactly your symtoms.
Perhaps you can get a piece of 1/4 inch plastic tube, put one end into your
ear, and listen for the hiss by running the other end along the underside of
the manifold. Either that or buy a stethoscope, but the first works quite
well and is inexpensive, assuming that you don’t shove it too far into your
Good luck - will be interested in the end result.
Sovewreign Series 1