engine backfires when first starting

Yeah they are supposed to be a pain to start.

Obviously you have thrown new plugs at it?? Once fouled a plug may not recover fully.

Replace the spark plug cap & try closing up the gap a little to 20thou (using fine wire plug?).

Does your model have lights hard wired on? Someone had bestowed to me the idea that the ignition vltg sags on RMX as it tries to power the lights etc. Never really looked at one, (you might have a different model). If so try pulling out the bulbs & if it gets better install a switch.