It’s been years since I had a motorbike but one of my colleagues regularly rides his Y reg BMW. Tonight it wouldn’t run properly. It started, then died. It wouldn’t restart despite turning over properly. No spark found when an HT cap was removed and shorted to the crankcase. Otherwise all electrics seemingly OK. Then weirdly, when he used the kill switch there was the biggest backfire! Scared me silly.

Can anyone suggest what was happening? Cheers, Ambrose

Dodgy earth somewhere in the vicinity of the coils? I’m presuming it’s an old y-reg r80 and so will have magnetos?

coil is a known weak spot – will have to take the tank off , usually the plastic coating is split and moisture ingress leads to missfires and eventually total failure, although at that age it should have failed by now

That’s what I wondered.
But why did it backfire? Charge dumped to the sparkplugs from the capacitor?

No exhaust full of fuel then it fired. Check the kill switch as well.

The kill switch backfire was an old trick, as said unburnt fuel.

Other than that, it’s a BMW, it’s supposed to break and do odd things

start here for R80 coil story http://redlegsrides.blogspot.co.uk/2008/07/joys-of-problem-diagnosis-on-airhead.html