2003 Suzuki VL1500, Replaced stator due to changing problems, now backfire and very poor running. Can't check changing system due to running issues. Ran fine before replacing stator, bout to pull my hair out.
Check the battery charge state...more often these days even a slight drop in voltage is enough to cause problems. next if it started running poorly since the stator was replaced I would try to check what the battery is getting while the motor is running. It should be around 12.5 vdc at idle....rising to about 14 or so vdc at maybe 5k rpms. If it isnt then I would suspect the reg/rec may be faulty.
Lastly I would be rechecking all the wiring and specifically all the connectors are fully home and correct and the stator is mounted correctly....I know this is probably a long shot but a spacer or similar in the wrong spot or left out may be all that it takes.