BSA Motorcycles 1950s

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The BSA Twin Carb Mystery

Since I bought an A7 in 1978 I wondered why BSA didn't produce a twin carb version of either the A7 or A10 pre unit twins to compete with Triumph. Triumph twins were always the sporty ones, leaving BSAs for those who were more interested in getting to work and back reliably, if a little more slowly.

In my naive enthusiasm for the marque I was convinced that Triumph owners wouldn't have had it all their own way in the sports machine stakes if the powers that be at BSA had shared my clarity of vision and fitted two carbs instead of just one – simple as that. But the texts were clear on this - Haynes, Bacon, neither list twin carbs on any of the A7 or A10 models, with the exception of the very early A7’s produced in the late 1940s.

I would have probably have just left things at except that soon after this I bought a copy of a BSA press photo from the UK BSA owners club which appeared to show an A7 with a rigid rear frame, central oil tank and twin carburettors – what was this? Occasional questions over the following years turned up nothing more than the odd owner who had ‘made’ a twin carb head out of a single carb one, with varying degrees of weld and success but this wasn't what I had in my photo. Then, in a job lot of A7 engines parts I bought as spares I looked at an engine which had been sold to me as a "genuine twin carb racer". With a cynical shrug I opened the crate expecting to see another example of a home-made job, but it wasn't. It was an alloy A7 head with twin inlet manifolds, it had obviously been manufactured that way and had a part number different from the one on my ‘standard’ single carb A7. Was this the same cylinder head in my BSA press photo?

University, work and a family followed after that, during which time the bike attained 'project' status. That is, it stayed semi-dismantled in my parents' garage until I owned a garage myself about 10 years later. This coincided with an email account and access to the web so I decided to explore the twin-carb cylinder head mystery again contacting BSA owners in the USA and Canada, swapping bike stories and 'what you got?' questions. To my surprise they told me about twin carb heads they had or had seen at swap meets. Different sources in different States were describing the same head as mine with the same part number – and it looked just like the one I acquired in my job lot and in the BSA press photo. So began research in earnest.

About a year down the line I now know that BSA did produce twin carb heads both for racers and roadster A7 and A10 models. If you want to know how and why, read on...