Properly decouple Arduino in and output with opto coupler from engine noise

[JohnRob]

I think the 1µF is marginal for really fast spikes, but it might be OK since this a motorcycle.

@ununnilium

I think big voltage spikes should not happen as long as the battery and rectifier are in good shape

If this were a vehicle I would vehemently disagree with you on this point. However for a Motorcycle, I don't have any experience.