Noisy Alton Starter

I suspect it is the sprag bearing failing. Watch the starter chain continue to go round after the engine has started. It should stop once the engine has started.

As Duncan says, your sprag clutch is not releasing straight away. They are fairly robust so it just might need cleaning. As you have a belt primary drive it means the sprag is running dry rather than being in an oily environment. I run my primary dry and sprag has been fine though. This might be a factor as the sprag is greased when new. I'd try taking it apart and cleaning / greasing it. Be aware that the Allen screws holding it together can be tight. Also double check stator to magnet ring clearance when reassembling. 

Hi - thanks for the guidance.

Prior to a strip down I gave the sprag a good squirt with WD40 and it worked - no more racket on start up. So it must just have been bone dry from the years of use.

Regards - Paul.

I run my chaincase dry Paul so occasionally I Squirt some oil down the back of the Alton sprag as when your riding the sprag is always running one way as far as I know. Its only when you start the bike that the sprag engages. I'm no expert but it seemed sensible to me to oil it sometimes as that won't do it any harm. I seem to be one of the few people not to breakdown on the way home from Aberfoyle.