Brian, it is generally accepted that in normal use the 6v starter will be fine with 12v, but take care if the engine is tight. At a slow cranking speed the current is higher, so if with 12v it only spins slowly then you could possibly damage the starter motor if you spin it for long. Short bursts with time to cool between might be best.

I think that it would be adviseable to disconnect the 6v feed from the starter entirely, making sure that there is 6v only, feed to the coil etc before trying to connect in the 12v.

Location: Oakley ,Hants

Many thanks Andrew & Derek, that all sounds like good advice.

Location: Near M1 Jtn 28