Hi all i have a 6v battery on my yamaha,when connected to a volt meter in idle tick over.it reads 8.5 is this correct many thanks.

I don't know if that is sarcasm or not?

It is definitely wrong, as the revs increase the voltage across the battery will too. If your battery is in good nick you need to replace your voltage regulator. Replacement is the only real test.

You could be looking at as much as 12 maybe 14 volts at high rpm. Goodbye bulbs, goodbye battery.

Yip mac... find where its located,, reg/rectifier unit,,, it should be 7.5 volt max... I think your bikes been tampered with , as u said a 12 volt battery was fitted.. Read this post,,,

http://www.team-bhp.com/forum/motorbikes/69718-yamaha-rx-100-queries-4.html

And this may help too .