This is what D W Munro (one of the A10/A7 design team) in his book BSA Twin Motorcycles, has to say on the subject of induction bias. “The two induction ports may be mathematically symmetrical, as far as the best inspection methods can ascertain, and yet there may still be induction bias” Munro then goes on to say that the problem was not universal, and that the A10 seemed to be more prone to induction bias than the A7, and although there were customer complaints they were relatively few. He continues “Never the less the makers (BSA) have made a serious investigation of the trouble, and as a palliative for persistent cases have produced a special anti - biasing gasket , part number 67 - 359 for insertion between the carburettor flange and the head.” It is most interesting I think, that Munro chooses to use the word palliative (derived from the Latin palliare, to cloak) or as the dictionary says to relieve the symptoms without effecting a cure. While I am quite confident that the anti - biasing gasket might work in some circumstances for some people, I am of the opinion that it was a cop out on the part of BSA. It is much more likely that the problem is caused by a combination of factors related to the magneto that would have cost BSA much expense and inconvenience to rectify. Inaccurate cam ring lobe machining, and poor alignment of the armature shaft due to the bearing housings being off centre for example. The magnetos were built down to a price and considerable extra cost would have been incurred by BSA if they were all inspected to make sure that all magnetos complied with the designed tolerances not to mention the extra cost that Lucas would have added to do the same thing. It was the custom in Midlands engineering companies in those days to pay their employees on a piece work basis, and piece work is not exactly conducive to producing consistent guaranteed quality and precision. The number of units produced was the overriding factor. Because my engine appeared to be running richer on one cylinder I tried an anti - biasing gasket and I noticed no improvement. I then had the magneto rebuilt with particular attention being paid to the cam ring (it did indeed require rectification) and it was also discovered that the bearing housing was in need of re - alignment. It transpired that the armature shaft was turning eccentrically. Since this work was carried out my engine runs beautifully and the plug colour is the same on both cylinders. Of course another palliative in bygone days for this problem (and it is still advocated by some people) is to split the difference in any point gap errors that are observed. Probably today with CNC technology this problem would no longer arise. Do you remember the work round on car distributers? The solution to eliminate the problem of point gap errors was to set the points up with a dwell meter.
John
