I would like to start off my telling everyone I'm new to thumper talk and dirt bike riding as far as that's concerned. I've been reading around the forums now for a couple weeks and the info I've found so far has been awesome! Please be patient with me as I try to explain my issues, thanks. I recently bought a 99 kx 250. The bike was is great shape when I bought it considering it was 14 years old. The only thing I've changed so far is the 13t front sprocket to a 12t front in order to make it easier for me to ride. After reading in this forum I would eventually like to swap the 99 tranny for the 98 tranny. Sorry, back on subject. I found after riding a few weekends that the throttle cable was in really bad shape so I replaced it. While I was doing that I replaced the air cleaner with a freshly oiled twin air filter. I then went out this morning to go ride and noticed the bike did not want to rev from 1/4 to full throttle. I immediately stopped the bike to figure out what was going on. After looking at it for a bit I decided to change the spark plug and put the original air cleaner back in. It took a while to clean up but the bike eventually started to run better, but not back to normal. I am looking for advice on what could be going on for sure. Is there a common issue with it running weird after oiling an air filter? The new spark plug didn't seem to change anything. I'm worried that its leaned out for some reason and I don't want to burn up the engine. Any advice or help with this would be much appreciated, and hello to everyone in thumper talk!