either A) have the bike be kind of old and generally running like crap
B) put a full exhaust/air filter on but don't rejet the carburator so its running way too rich or lean
C) get a power commander and turn the tables way lean or way rich

so in other words if your bike shoots flames its usually a bad thing, if you have an exhaust and it spits a quick little flame when you let off the throttle at high rpms i would be too concerned, but being able to shoot flames on command would mean a bad air/fuel mixture