The factory,   if I remember , set the Idle mix screw at 1 1/2 turns open, I'd start there . You then slow or speed up the idle (knurled )speed  screw as low as you can go without stalling ,so you can hear the speed change ,and adjust the mix screw open until you get the highest speed . then reset the idle speed screw to 1000 rpm to restore your oil pressure. If the mix screw ends up 3 turns open it can vibrate out ,you'll need to change to the next larger pilot jet. If the bike isn't stalling , but putters/ gargles when you close the throttle fully at any speed the bike running (moving) ,That is normal for a single cylinder bike, loud backfires are not, and may mean you need to clean the TEV valve and it's passages .

Aftrfire is sometimes caused by air leakage in the areas of the boot between the carb and head  , and exhaust, at the header or the header/muffler . You can cheat at shutdown , by placing the bike in neutral ,allowing it to fall to an idle , and pulling the choke out just before hitting the kill switch. this sends a fuel mix so rich into the exhaust header that there's not enough air to support combustion- no puff ,no afterfire !