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My spark plug on my 2001 yz125 keeps fouling

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I have had a lot of trouble getting the bike from a dead stop to moving in first gear without massive bog and stalling. Once the bike is rolling, shifting through gears is no problem. I pull the plug after 10 minutes of riding around and its wet and oily. I have repeated this process many times and i keep getting the same results. So i replaced the Spark plug and it fixed the problem for 10 minutes and then it became fouled again. I have adjusted the idle speed and that doesn't seem to help the bike. Does this sound like I need to clean the carb or what other problem could it be? The guy i bought the bike from just put a new top end in and he is running a 20:1 mixture. There is also a lot of black gritty discharge from the exhaust. I'm knew to dirt bikes so any advice helps. Thanks

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You need to change your jetting. The bike is running too rich. Start by leaning out your pilot jet 1 step at a time. Get the OEM service manual and it will explain everything.

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Ratio should be 32:1.  Start with stock jetting.  Yamaha yz jetting is usually pretty good unless you are at a high altitude.  Usually it is just a matter of one clip step, and/or a pilot jet smaller.  My 06 125 runs best with the half step leaner needle and an 8 heat range plug in the woods, but a 9 for the track.  My 03 runs best with the stock jetting and 9 plug.

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did he just say he put a new top end in or do you know he did?   most people rebuild the top end when they buy  a used bike.... I did, the guy I bought it from told the truth it was a fresh piston in it  If I was you remove and clean the carb really good, set to stock settings, change oil mix to 32:1, 20;1 is to much IMO.... might as well throw a piston in it while you are at it. its really easy. just take your time. same with the carb, its simple.

Edited by yz250474

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I will reset the jetting to stock and i just drained all the gas out. The guy showed me receipts for the top end rebuild so i know thats accurate. how difficult is resetting the jetting and  Thanks for the replies 

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I will reset the jetting to stock and i just drained all the gas out. The guy showed me receipts for the top end rebuild so i know thats accurate. how difficult is resetting the jetting and  Thanks for the replies 

you need a manual for your bike to know what jet sizes, float height, needle clip position, air screw etc. its really simple once you understand the basics. its some video's on youtube. but if you are sure it has a fresh topend, you could just change to a 32:1 premix (I think that's what Yamaha recommends for a 125) it will richen your fuel mixture a little. (less oil=more gas) see how it runs and go from there. I have about 8 tanks thru my 250 with yamaha's stock settings, never fouled a plug even with excessive idling.

Edited by yz250474

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are you sure the choke is off and the air filter is clean?

Good point!

I will modify what I said before. Assuming your air filter is clean, choke is used properly, and ratio is 32:1, the stock yamaha jetting should be really close, unless you are riding in very unusual altitude/temperature conditions.

One last thing. Are you riding in the correct rpm range? Sometimes beginning riders switching from 4 stroke play bikes don't use enough rpms. Go to YouTube and view some riders ripping around on yz 125s. That should give you an idea of the rpm range.

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I certainly do need to learn how to rev a 2 stroke and ride properly but i dont think that the plug should just instantly foul that fast and it may be due to the guy who put 20:1 in and with that ratio i should be constantly revving.  

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So i put the new fuel ratio in (32:1) and it seems to have fixed the problem. If it starts to act up again i will do a full carb clean and reset jets. Thanks for all the advice, I really appreciate it 

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