Extract from Safety Fast!
Electronic Ignition Conversions

The following relates to my experiences in fitting electronic ignition to my 1973 MGB. I am sure that the general findings and observations would apply to other M.G. models. (Reprinted with thanks, from "The Breed", the magazine of the M.G. Car Club Cape Town Centre).
by William Ribbans

The Basics
First a bit of basic background information into the operation of a distributor generally, and a B chrome bumper distributor specifically.

Electronic ignition conversions for the MGB are quite attractive because they are not too expensive. Once installed, you will never have to set the distributor points gap again (not very easy in the A & B series engines) and you will have the claimed benefits of extra economy and power. They also work quite well in spite of any distributor spindle wear.
There are, as far as I know, two electronic ignition systems on the market. The Lumenition system, with its external "black box" that must be mounted in the engine bay, and the ignitor. I prefer the ignitor, not for any performance benefits, but for its aesthetic appeal (not a very sound technical reason I have to admit)!.
Once installed, the only visible signs that you have installed the ignitor are the red and black wires emerging from the distributor and leading to the positive and negative terminals on the coil. (A standard points set up has only one white and black wire from the distributor to the coil).

We all know that the distributor must distribute the spark to the spark plugs in the correct order and at the correct time. Two factors affect timing; as the engine speed increases the spark needs to be advanced relative to the piston position (performed by the centrifugal advance system of weights and springs inside the distributor), and also, at part throttle when the fuel mixture is leaner than normal, additional advance is also needed (achieved by the distributor vacuum advance which is connected to the carburettor body between the throttle disk and the SU piston).
The specification of the Lucas vacuum advance unit is stamped on the body near the vacuum pipe take off. For a chrome bumper B the number is 5 13 10, which means vacuum start at 5° HG; ends at 13° Hg and advances 10 degrees in the distributor (i.e. 20 degrees on the crankshaft). The only real problem experienced with the vacuum advance, which is a sealed unit, is a leaking internal diaphragm which is quite easy to pick up. The whole unit must be replaced if faulty. No problems here!
The centrifugal advance is a little trickier. The advance rate is controlled by 2 springs attached to the centrifugal weights which are situated under the points mounting plate. Chrome bumper 25D4 distributors (with the serial numbers 4897 or 41288 stamped on the side of the body) have the following centrifugal advance specifications;

at 600 rpm 0-6 crankshaft degrees
at 700 rpm 4-9 crankshaft degrees
at 900 rpm 7-11 crankshaft degrees
at 1600 rpm 13-17 crankshaft degrees
at 2200 rpm 18-22 crankshaft degrees

The graph of the centrifugal advance curve shows the upper and lower limit of the above specification. You can see that initially the rate of advance up to 800 rpm is quite rapid and thereafter slows down for the second stage up to 2200 rpm. Removing the points mounting plate with its base you will clearly see the 2 springs. The light spring control the initial advance and the heavier one for the second stage.
You can also confirm that you have the correct cam spindle which should have a '10' stamped on it (chrome bumper cars). The '10' indicates a 10 degrees distributor advance cam spindle giving 20 degrees advance on the crankshaft for the centrifugal advance only.
The centrifugal timing on these chrome bumper cars (still with points installed) can be simply checked by setting the static timing to 10 degrees before TDC (top dead centre), disconnecting the vacuum advance and then rechecking the timing with a timing light and seeing if you get the correct advance as required by the specifications stated (i.e. at 600 rpm idle you should have 13 degrees before TDC - 10 static and 3 centrifugal).
Finally check the total centrifugal advance by revving the engine to approximately 2200 rpm with the vacuum advance disconnected and checking that you have approximately 30 degrees total crankshaft advance (10 static and 20 centrifugal). Should you be getting more or less than 18-22 degrees centrifugal advance, you should check the gap between the cam spindle and its stop.
The gap should be approximately 4.0 mm (best to check it with a drill bit). If adjustment is required, a bit of filing or careful welding will be required.

The Problems
Enough background information, now on to the problems that arise when fitting an electronic ignition system. I do not have any personal experience with the Lumenition system, but would imagine that most of my findings with the Ignitor would still apply.
The manufacturers of these electronic systems all claim that you will get extra power and economy but what they donít tell you is that to achieve this you will have to recalibrate your distributor (no easy task). After fitting my electronic ignition, I did the checks described above.

The Solution
After much head scratching, reading of literature, pondering and lots of trial and error, I figured out what was causing this unusual behaviour. It boiled down to one simple fact - friction, or more precisely the lack of it!
Let me explain - with points installed, a reasonable amount of force is required to overcome the friction of the points-cam-follower over the cam lobes of the distributor cam spindle. This friction also assists the 2 centrifugal springs to return the centrifugal weights back towards their zero advance position. Now what was obviously happening was that the weights were not returning to their zero advanced position with the engines switched off making it impossible to set the static timing correctly. Also, when the engine was started, the weights would advance the timing too rapidly as they now only had the springs (and no help from the friction of the points) to hold back the rate of advance.
So, instead of only having about 3 degrees of centrifugal advance at 600 rpm, you had a whole lot more which also resulted in the full 20 degrees being reached by about 1200-1500 rpm. Assuming you had set the static timing correctly to 10 degrees (the original factory specification) this would tend to give you a lot of pinking. Clearly what was called for was a change of springs. I scrounged some heavier ones from some old Lucas distributors at a scrap yard. The spring thickness controls the rate of advance and the length controls the rpm at which the spring starts to work. Getting the length correct required quite a bit of trial and error and in the process I lost count of how many times I had the distributor in and out of the car. The length I arrived at should be fairly close to what is required with maybe slight adjustment for distributor manufacturing tolerances. The spring specifications before and after are shown below.

Original spring specifications;
wire thickness length
1st stage (thin spring) 0.4 mm 15.0 mm
2nd stage (thick spring) 0.9 mm 17.0 mm
Revised spring specifications;
wire thickness length
1st stage (thin spring) 0.7 mm 16.5 mm
2nd stage (thick spring) 1.2 mm 17.9 mm

These changes now give me a distributor which has to be set statically at 13 degrees before TDC or dynamically also at 13 degrees before TDC at 600-800 rpm with the vacuum advance disconnected. Also, maximum centrifugal advance was adjusted to give 30-32 degrees before TDC at 3000 rpm (13 static and 18 centrifugal). This was on the recommendation of Peter Burgess in his excellent book called "How To Powertune MGB 4-Cylinder Engines". The revised curve is also plotted on the earlier graph. You can see that the 2nd stage spring now starts operating from around 1500 rpm.

Another unexpected benefit of these distributor modifications is that the "run on problem" that seems to affect most Bs, has stopped. I am unable to offer a scientific explanation for this but I am very happy with this unexpected result.
I hope that this article has shed a little light on the murky operations of a distributor and the associated problems arising when trying to maximise its performance after fitting electronic ignition.

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