If you work at the prolem with logic looking at each thing you touched from when it ran to when it didnt im sure you will find the problem. I had the same problem with no start and just backfires and i retraced what a did and with a little luck found what it was that was buggered. Turned out it was the one thing that i had touched since the bike ran!. So thats why the guys here are saying that looking at the carb should be the first thing you do. My bet? you dislodged some rubbish in the carb when you changed the flat needle that either got into the jet or stuck the float vavle slightly open making the engine flood. Just have a look at all the things youn have changed or touched, You never know it migth be just that you moved the wiring harness and made a dodgy connection stop working fully.
But it may just be as simple as a fouled plug, remember you can have a plug that sparks but wont start as when the piston comes up its much harder for the engine to spark and it finds and easier route to ground further up the ceramic.
keep patient and methodical and you will figure the problem out
85 Tri-Z "Rumble Bee"
85 250R "Red Rocket"
83 200X "Flat Tracker"