Do you have a decent ear for tuning? Liston for pinging, or decel pops… if you hear them you may need to rejet a bit. Honestly alot of quads come rather lean right out of the box. Do a plug chop: warm up the machine then slowly get to 4th gear, and hold your thumb to the bar from idle to redline, then pull in the clutch and hit the kill switch. DO NOT start it again, let it cool and take the plug out. Read the plug. If you dont know how to read the plug… ask for help.
And yes changin an air filter can throw a tune off alot actually. Stock 04-05 yfz450’s were lean from the factory. Just taking the lid off and pulling the pee shooter on my machine, I copped the plug and it was WHITE. I ended up bumping up the main, and moving the needle one position richer and it was nice.