The vacuum unit works to advance the spark on part throttle in order to improve efficiency and fuel economy when cruising/at steady speed. At full throttle the manifold vacuum disappears and so the vacuum unit gives no advance. Hence it can be seen why a vacuum unit which advances too far/too soon will give the very distinctive symptom of pinking on part throttle which disappears or diminishes on full throttle.
Note: Sometimes owners are unaware that this is the case, as they (wisely) back off the throttle at the onset of pinking to reduce engine damage. For diagnosis, you need to grit your teeth and put your foot to the floor to see whether the pinking gets worse (in which case it caused by the mechanical advance) or disappears (the cause will be the vacuum advance unit) or remains but reduces (both).
If this test reveals that vacuum advance is the cause, by far the most common reason this is the case is when the vacuum unit has been replaced in the past with a wrong spec (but identical looking) second hand one – usually from a Mini. Mini ones advance further and sooner than an MGB one. The test to confirm is to disconnect the vacuum unit and see if the problem goes.
The vacuum unit specs are marked on it. 25D distributors (the ones with the knurled adjustment knob fitted up to about 1972-73) have these markings around the neck where the vacuum tube from the carb attaches. There are three 1 or 2 digit numbers separated by dashes, for example the MGB is 5-13-10. The first figure is the vacuum at which the unit starts to advance, the second is the vacuum at which the unit finishes advancing (both in inches of mercury) while the third figure is the distributor advance in degrees. Thus the vacuum advance on the MGB 25 D distributor:
- Starts at 5 inches of mercury
- Reaches its maximum advance at 13 inches of mercury
- Has a maximum advance of 10 degrees at the distributor (which is 20 deg at the crank of course)
The vacuum advance specs your car should have are given in its workshop manual. This problem is confined almost exclusively to MGB’s with 25D distributors as these had quite a lot if initial mechanical advance compared with A series engines and later MGB’s.
(Note however that replacement distributors are all 45D, some of which are to the same specs as the 25D.)
Any remaining pinking should be followed up as described on the detonation and combustion pages.