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MG MGA - distributer type

I seem to have developed a lot of rattling coming from the area of the distributor noticeable at tickover so I've taken it out for a look and found the following:
The weights are quite loose and do indeed 'rattle around even when the spindle is at its 0 degree position. Its got 15 degrees stamped on the cam -is this correct for a Mk2?
The body is stamped 40761A -is this correct for a Mk2 and does anyone know what the A signifies? I have a spare distributor stamped 40561L with 13 degree cam and wondered if that might be suitable if the original is worn.
J H Cole

JH. There is a distributor data base located at the following-
which may be of use to you. As I remember it, the figures are given in distributor rpms and degrees of distributor advance--multiply times two to get the figures that would be seen at the crankshaft.

As to the distributor for the Mark II engine, I do not know what the original distributor specification number was. It should be contained in the data base listed, but may take some digging to find. My copy of the factory workshop manual does not list a special distributor for the Mark II cars. It lists that the 1600 used the "Lucas
type DM2, Later models DM2.P4", but does not note the specification number of the distributor, nor does it list the mechanical advance curve required for the MGA.

I believe that the specification 40510, often followed by an alphabetic suffix (indicating a minor manufacturing change which did not change the basic specification of the distributor) is the most commonly found MGA distributor.

Les Bengtson

J.H. the 13 degree mechanical cam is pretty much fine for the B series engine. That equates to 26 degrees of mechanical advance at higher revs and so to get to the required figure of abot 32 degrees max advance you need to time this distributor at static as 6 degrees BTDC. distributors with less degree cams than yours can get to much advance at mid range and with modern fuels (unleaded 95) can get pre ignition/pinking.
Bob (Robert) Midget Turbo

The 40761A distributor was fitted to MGA Mk2 low compression engines, whereas high compression Mk2 engines used the more common 40510 with an H suffix.
The 40761A distributor in my Mk2 also has 15 degrees stamped on the cam. As a matter of interest all Mk2's sent to Australia as CKD's had low compression engines.
Garry Kemm

Gary, my distributor appears to be the same as yours but I am surprised at how light one of the springs is (primary /secondary?) since it does not even keep the weight against its stop. It could be that this is not the correct spring -shown with white marking. Piet Olyslager in his handbook gives the centrifugal advance for the Mk2 as 24 degrees and the 1600 as 12 degrees -why should the Mk2 be double?? There is no reference to any variation between Mk2 high and low compression engines. I must say that in other technical areas of the MGA one can often get chapter and verse but when it comes to the Mk2 distributor and its weights and springs its shrouded in mystery, or at least to me it is.

J H Cole

JH. The stronger, secondary spring, comes into play at higher distributor rpms. Curves set up like this will show some degree of early (low engine rpm) advance fairly rapidly. Then, the advance will slow down and become flatter as the secondary spring comes into play. Quite common on MGs.

As to the difference in mechanical advance, you would have to ask the authors who listed the numbers how they were obtained if you want the true story. I would note that there are two methods of listing the mechanical advance:

The first is as the numbers are read off the distributor test machine. The distributor test machine rotates the distributor and causes it to spark. You zero the protractor at very low speed, giving you your basic zero, then, chart the mechanical advance at various distributor speeds. This is the most common form of initial figures.

The second method is to either read the mechanical advance off the engine, or to convert the distributor test machine results to engine results. The distributor rotates at one half engine speed. This means that the engine speed is twice that of the distributor and the degrees of advance seen at the engine crankshaft pulley are twice that seen at the distributor. Many people prefer to give the figures as seen at the crankshaft pulley because this is how most people will be using the numbers.

The figures you give, 12 degrees of mechanical advance and 24 degrees of mechanical advance, may be an instance of one being given in distributor degrees and one being given in crankshaft degrees. But, this is only a guess on my part and the authors of the articles, and the sources they used in writing the articles, will give you the best information.

Les Bengtson

This thread was discussed between 13/12/2008 and 20/12/2008

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