Finally had time to work on my bike yesterday.
When I removed the front sprocket I obviously had to remove the speedo rotor that attaches to the front sprocket shaft. It only had 3 prongs on the rotor - I thought 'Looks strange'... I now realise there should be 4, which is why my speedo was 12mph slow (on standard gearing)
I have a Speedo Healer and I've got it set to give a true reading so while the missing prong isn't a current problem it might be something that will be a pain later.
So if anyone has a spare one kicking around...
