Tension trouble

Pete Whichello asks for a degree figure for the ignition timing on his recently acquired 1954 BSA A10, as all he can find is a piston depth measurement of 11/32in before TDC and he’s wanting to retard by 2° for unleaded fuel.

Strangely, although I don’t recall a problem finding this information when I had my A10, I couldn’t find it now. The BSA service sheets, owner’s handbook, Haynes manual – even Roy Bacon’s excellent BSA Twins and Triples book all quote the linear measurement.

However, I’ve never retarded any of my bikes to run on unleaded and it doesn’t seem to make any difference. Before digital ignition, timing was a relatively especially on twins where inaccuracies in internal timing can leave the spark 5 or 6° out side to side. Besides, compression ratio is important to timing and there’s no telling whether the BSA is still running standard pistons.

The good news is that the same slack manufacturing tolerances that make old bikes less reliable than new also make them less fussy, so I would see how the bike runs on unleaded – and so long as it doesn’t ‘pink’ or overheat, I’d leave it well alone.

Inspiration finally struck about my own BSA timing, though – I used to keep a board with service info pinned to the shed wall. I no longer own most of the bikes on it, but the A10 is on there – I used to set it at 38° fully advanced, but don’t ask me what that is in inches...