Running Lean

My KX550 is straight stock. Running down the interstate with the tack at about 5500 RPM, I twist the throttle. The bike feels like it sags and then pulls away strongly. If I do the same thing at 4500 RPM it just pulls away strongly.

I think the main jets are lean. So the question is why? And what could make it happen. Stock exhaust, stock, carbs, stock airbox????

I first noticed it about 8 months ago after replacing the airfilter. I had owned the bike about 4 months and was doing a tune up. New plugs, air filter, such as that. When I looked at the old filer it was oil soaked and had big holes in it.

So I am wondering, should I rejet, or figure out why it is running lean. And what are the things that could cause the running lean assuming the jets are correct.

Thanks.

ok...so what bike IS it?:lol: ...a "KX" or a KZ":whistle:(kidding)
Assuming it's the KZ...and the air filter had holes in it?That might have something to do with it,but what did the old plugs(or even the new ones for that matter) look like when You pulled them out?
Have You had the carbs apart,and checked float-height?
You need to get into the carbs,so You at least know what jets are in there...