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Yz125 2001 Fouling Plugs Constantly

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Hi how are you, I just purchased a used 2001 Yz125 about a month ago, and this past week i have been having a huge problem with fouling plugs.  When i ride it easy, it never fouls but when i take it into the powerband, about 75% of the time the bike makes a horrible sound and just bogs way down.  I take out the plug and it has oil all over it, so i clean it off and it fires up fine.  However, i never fouled a plug in the first 3 weeks of using the bike, so something that i recently have done to the bike must have messed something up, but I dont know what.  First off, I put a new spark plug in, a NGK BR9ES, and secondly, i took the carb out and cleaned out all the jets and the float bowl, and put everything back exactly how it was. After i did this, the bike has been fouling plugs like crazy.  I am mixing 32:1 Yamalube premix, and according to the seller, the bike had about 40 hours on the top end, bike starts first kick. What is happening with my bike? Thank you for reading my post and your help is greatly appreciated

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Yeah I'm looking to rebuild the top end within the next 3 months, but what is the best spark plug for a yz125 running 32:1? I put my old plug back in (cleaned) and it hasn't fouled yet. Its an autolite 4062 but I want the best NGK plug for it.

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Yeah I'm looking to rebuild the top end within the next 3 months, but what is the best spark plug for a yz125 running 32:1? I put my old plug back in (cleaned) and it hasn't fouled yet. Its an autolite 4062 but I want the best NGK plug for it.

You said once it hits the band, it will foul? I would think that would have to do with a main jet being way way too rich. Possibly your needle. Make sure the jetting is stock and then adjust accordingly.

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If it runs fine with the plug ya got....run it.  Have ya done a plug check?  Scream it, kill it, check it....what's it look like?

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If you ride in the woods go with a B7ES. Make sure you set the air screw correctly as well. With a stock pilot jet it would be between 1.5 and 2 turns out depending on the weather. What are you running for a main jet? I'd try around a 410.

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I would do the top end sooner then later. They always blow when you try for that one last ride.

I'd also see how much Trans oil comes out, put in the proper amount, ride for a few hours and drain it. See if any disappeared. That would be a leaking Trans side crank seal and would foul plugs with Trans oil

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Hey guys I appreciate your help. I just changed the oil and the same amount came out after about 20 mins of riding , so I don't think the bottom end has to do with this. I have also been running my old spark plug, the autolite 1042 or whatever but I just ordered a couple NGK br8es instead of br9es(the one that fouled constantly) I'll see if the br8es work if not I'll try b7es. Otherwise I'll get into the jetting which I'm not too familiar with. Thanks again everyone.

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Compression doesn't seem too bad. I don't know what the reeds look like inside but they are v force 3 reeds. I definitely need a top end soon though.

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a chipped or cracked reed will cause this, easy to pull and check them, take a peek at the piston too.

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