06 KTM 450 sx backfires

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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.