Yes the flame coming out is a good indication of spark, however that spark may be extremely weak and not firing EVERY time and hence when it does decide to fire a nice spark it burns all the fuel sitting in your exhaust. Just toss the plugs dude and go get new ones from auto zone or wherever. I've kicked my ass off trying to start dirt bikes, pulled plugs, cleaned them and still nothing but backfires every so often. Put a NEW plug in it and it fired right up. New plugs + starter fluid = your bike running