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2002 yz250 surging at idle

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So my new to me 2002 yz250 has started giving me problems. The bike is mostly stock except for an fmf gnarly and 182 main jet. Previous owner never really rode it so didn't do much maintenance if any on it. It ran great on my last ride but the other day I started it up without a problem but immediately noticed it was surging at idle almost like a miss and was idling just a tad high, lowering the idle would help but it was still surging. Took it for a ride down my street and noticed a hesitation from closed to 1/4 throttle but ran great past that. I went back home and tried messing with the air screw which didn't help the idle, it started dripping coolant out of the overflow so I decided to shut her off.

     Now I'm Amost certain it's an air leak and know I need to do a leak down test but I haven't gotten around to making one yet. This is only the 2nd 2stroke I have actually worked on so any advice would be great. I was hoping someone else has had similar symptoms and could point me in the right direction. Really hoping it's not my crank seals as the last case I split was such a headache and I never got it going again. Help me I'm poor!!!

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If it's an air leak from the crank seal it's on the flywheel side. If you remove the flywheel and stator you can replace that seal.

Look at the airboot/intake for cracks or loose clamps. Also check that the carburetor is fully seated. Also check the condition of the O-ring on the pipe.

PS. This is the 4 stroke forum

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Thanks for the input ohiodrz. I spent hours researching last night and saw that my symptoms are indicative of a stator side crank seal, after reading how to change crank seals I was very relieved it's a fairly simple fix. 

     Now another interesting thing is my pipe does have some spooge coming out around the flange. During my research I did not see anyone mention that so I kind of overlooked it. Gonna start with the pipe o-rings since I know they are leaking spooge.

    before I realized it's an air leak I went ahead and bought piston kit thinking it was a head gasket which is a good and bad thing I guess. Damn 2 strokes. Hella fun 2 ride not so much fun to diagnose. At least they are cheap and easy to work on.

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Before you start tearing the engine down and replacing parts, start with the simple things. Make sure all maintenance items have been addressed, like the air filter (clean and oil properly), correct gear oil level, good quality pre-mix at correct ratio with good quality 91+ octane gas. Then make sure the carb is jetted and tuned properly (I'd go back to totally stock settings and start from there). You are going to have a very hard time chasing a problem if you overlook the simple things that need to be maintained anyway. Something as simple as poor quality fuel can cause all kinds of issues.

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Roger that. I replaced the air filter as soon as I got the bike, rebuilt the carb with new gaskets but all my Jets looked good and are stock except for the main (182) which only effects wide open throttle so I'm not really suspecting that's a problem. 

The gas in the tank is a couple weeks old and the bel-ray pre-mix I got from the PO was hella old. Unfortunately here in Maui, HI 91+ is hard to find. Most stations only go up to 90 so that's what I'm using ATM. Spark plug is an iridium and has one ride on it but I did notice an uneven burn pattern like it's only firing on one side... I'll probably switch back to the stock to see if that helps.

Just took the pipe off and noticed it wasn't sitting flush due to being bent and crooked. I bent the flange back so it now sits flush on the head but it is still fairly loose when sliding on so I'm suspecting old worn out o-rings and retaining springs. Gonna replace ASAP. The bike did run slightly better with a lower idle but still surging from idle to low throttle then hits the power-band hard. Are the pipe o-rings that crucial to sealing the engine from air leaks?

Last thing I did after riding it was pull the ignition cover to see if there was pre-mix or at least an odor of pre-mix in there which there was not. I ran out of wrench time so I didn't pull the stator to visually inspect the seal. Although when I wiggled the flywheel up and down I did notice a very very small amount of play in the main bearing, and I mean barely any but I'm paranoid it could be an issue.

Gonna do a leak down test as soon as I make a tester and go from there.

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Another thing that's throwing me off is it ran great on the previous ride which leads me to believe something failed or is beginning to fail

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Well apparently there should be absolutely NO play in the main bearings.... This &%$#@!ing sucks cause I can't really afford to have the bottom end rebuilt unless I do it myself and I'm not too eager to split the case.... Is there anyone out there with a Tiny Tiny bit of flywheel play that doesn't have problems? Im just having a hard time dealing with the fact that my new to me bike which wasn't cheap (1800) is already needing major work after only two rides. FML

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