Sounds lean to me. I would check these things out, make sure that the air filter is clean, put in a new plug (gapped properly), drain out the old gas in the tank and put a gallon of fresh 91 octane. Now try the bike, if it is still backfiring, I would pull the carb off and clean it thoroughly pulling the main jet, pilot jet and check what position the clip is in. I would check the owners manual for stock carb settings (if you don't have a manual get with your local KTM dealer) and adjust the carb back to stock. If you are in a higher altitude, check with your shop and see what they are running in their bikes for mj, pj and clip position.
ktm 450 sx
The KTM sx is fast but the yfz is the best for what I do, what kind of riding do you do?
The 450 SX is a single cylinder 4 stroke, so no you do not mix gas.
Seat height on the 2010 450 SX-F is 38.78 inches
1.35 L
KTM recommends 1.35 L of Motorex Power Synt 4T 10W50.
KTM 50 tops out around 50-52 mph my 06 ktm 50 hits 55-60mph
The sx is for supercross.
The bottom back side of the cases by the countershaft (facing the rear wheel).
240 ML of 5w fork oil.
The KTM 360 SX is still a good bike, like all other KTM's dependable but parts are expensive when you need them. Here is a link that has pictures of the 360 SX: http://www.bikepics.com/ktm/360sx/
The 06 65 SX takes .3L (or 10.147 ounces). KTM recommends Motorex Top Speed 4T 15W50.