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When to replace top end?

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Ive been riding 4strokes for all of my life and I just bought a YZ125 this summer. It is currently not running because I keep filing plugs. It is at the point it wont start until u clean the plug everytime then it wil lbog and shut off. So i went and bought a hotter burning spark plug to see if that will work. If it doesnt does that mean I need a new top end? How can you tell when you need to replace it?

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you have to pull the jug off to get a good look. Its very easy. Wash your bike.Drain your coolant.pull your tank and seat off.take your plug out.pull the top off.(4nuts)now take the jug off.(4nuts)Now you can inspect the top end and connecting rod. Look for wear spots (they may be shinney spots or discolored) Your cylinder will have factory marks on it. Alittle carbon is no big deal. Piston kits are cheap. piston ring and gasket new coolant 150$ if that.torque the bolts back up. should take about 2 hours. then your good for the season.

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it may not be a top end. your jetting could be way off. is it stock jetting?? if so go down a size on the pilot, and the main. what premix ratio are you running?

what year is the 125??

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oh ya. sounds like your jetting is plugged.clean the carb.jet if need be.clean out your power valve. put in the stock plug whatever it is. and get a manual.and new gas

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what year is the 125??

its a 2004 and is it hard to re jet a bike cause i dont have a clue on a two stroke?

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id just try messin with the air screw on the outside of the carb first....if not dig into that carb....(make sure air filter is clean before all this)and clean the outside of the carb before diggin in

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I have a 2004 yz125 too mine runs fine heres the jetting I run, try these with your fuel mixed at 32:1 and it should run like new. But check the top end, clean and oil the air filter, new plug and clean the carb. I run autolite plugs they seem to run alot better than the NGK BR9EG. The autolite plug is about $1.44 heres the part number 4062.

Main Jet- #390

Pilot Jet- #40 stock

Air screw- 2 turns out

Needle clip position 3

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