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99 KX 250 top end cutting out

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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!

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Was the old air filter dry? A oiled filter will make the engine run richer. You need to verify the jetting in the carb.....and the A/F screw setting. Put you new oiled filter back in and ride it..when it seems to be cutting out reach down and pull the choke knob up..if it  helps then it is lean..If it loads up worse then it is running rich.

 

BTW welcome to TT and congrats on your bike. they are great machines.

Edited by Blackwoodz

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Thanks for the reply.  Yes the old air filter was also an oiled filter.  It makes sense the new filter could cause me to need different jets but putting the old one back on and it still was having issues makes me scratch my head.  It sounds like it starts misfiring but then it cleans up some times.  I thought about what I did a little more and now I'm wondering about that throttle cable I changed out.  The boot on the top of the carb (on the old cable) was crack and stretch.  The new one is sealing around the top of the carb.  Could I have been getting air through there enough to cause issues?  I might be stretching.  Thanks again for the help!

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The air screw is set 1.5 turns out from slightly seated.  I had to run it before 2.5 turns out so I changed the pilot from the stock 45 to 42.  Ran awesome!  I've had the whole bike torn apart now looking for things.  Checked the piston from the exhaust and intake port and all looks fine.  This thing was an animal and it never spooged or had any issues.  The only things I changed since last weekend was the throttle cable (it had the cracked boot on top side of carb) and the air filter.  Temperature is changing here rapidly though getting ready for winter.  One weird thing though, I verified the jets when I got the bike and notice the pilot was stock(45) and the main was wayyy oversize(175) for my area.  I'm close to 5000 feet elevation.  The stock main was 158 that sounds off doesn't it?

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Just an update,  found out it was because I pushed the needle into the slide to where it bent the clip and raised.  I think I might have finger fumbled the slide and needle assembly.  Stupid stupid stupid.  Thanks for all your help Blackwoodz.  Now I have to tune a new boyesen rad valve.

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