One side back firing

[madcb450k]

I have a 71 CB 450 K4 and it all of a sudden started backfiring out of the left side. I don't know what caused it and I have no idea what it might be.
Any suggestions on where to start?
The carbs are synched so that was where I started, but I am unsure why only one side is backfiring.

[Xmotorcycletech]

It could be a lean missfire ( idle mixture circuit) Fouled plug on that side or a points issue. Does it backfire all the time or just on deceleration?