04 1200 Sportster - backfires - cuts out - fouls plugs

NGK plugs dont stop, especially all at once, so there is another problem.
There are some resisters in the plug caps that can fail.
Are the plugs carbon fouling?(dry black) if so make sure the air filter is new, and oil is changed.
Does the enginge blow smoke or use oil?(wet black)oil fouled may mean rings.
Fuel fouled may mean the jettting is wrong or choke stuck open
Its the battery good(new) and charging ,put a multi meter across the terminals 12v stopped, 14v running approx.
it always helps to drain the carb bowls every few weeks
If exhaust is not standard, the jetting may be wrong.
does the fuel tap work correctly? only use the run or res positions never pri
A short in the kill switch or a worn ignition switch can cause a miss or backfire