Backfiring during starting might be caused by a stretched timing chain.
Since the ignition is COP type ignition, there's no distributor. So a worn timing chain won't have an effect on ignition timing.
But a worn timing chain can have an effect on valve timing.
Since the engine doesn't have a distributor, you can't use the old method of watching the rotor in the distributor to see how much slack is in the timing chain by turning the crankshaft.
Instead, you have to remove one of the valve covers to observe valve movement to check for a stretched timing chain.
All you do is turn the engine over by hand at the crank bolt in the normal direction while watching any valve movement.
Then turn the engine over by hand at the crank in opposite direction.
If crank can be turned more than five degrees in opposite direction before you see any valve movement, the timing chain is stretched.