My suzuki intruder is missing backfiring on the

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.

Remember to look for vacume leaks. I seem to recall this model uses carbs? If so check the CV slide components also. All trouble shooting mistakes come from ignoring the simple and throwing parts at a engine. Many problems are also caused by using aftermarket exhaust or air intake parts. Suzuki engineers are much more qualified to make a bike run its best vs the typical dealership or other mechanic. Unless a dyno is available with a exhaust test unit it is not a great idea to make performance mods to these complex systems.