At 55 mph my intruder 1400 backfired and lost spark.

Could be contaminated fuel, or bad spark plugs.

POSSIBLE RUNNING TOO RICH AND IDLED TO LOW

I also have a 1400 intruder, it backfires, and cuts off often at idle. I have tried many things.. HELP

That could be the spark plugs causing the run on. The rear piston runs hotter because it is heated by the front one, so try a cooler plug.

No, backfiring is almost always the mixture, not the plug and probably not the wires unless they are getting crossed. I would assume that in this case it is probably from the mixture being too rich. I would say the choke may be staying on too long.

Check spark plugs or carb jets .could be fuel or electrical problem

Did you sync the two carbs? If not the rear may be getting all the gas from the rear carb and none to the front. Twist throttle engine off and see if front carb cable is pulling the carb open.
Also unscrew the spark plug boot from the front spark plug wire and trim 1/8th inch off the wire and screw the boot back in. The plug wire end may be corroded on that wire.

is ther e no fuel mixture screw u can try adjusting

sounds if you have a burnt valve/s somewhere