Question about 2003 Yamaha YZ 125

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I have a 2002 yz 125 and i just recently had it rebuilt both top and bottom end. it was running fine a couple rides then all of a sudden it started to act up when i would take it up to high end and it would die. as soon as the motor stops fully turning it starts right back up in oone kick. i talked to dealerships and they all said electrical, carb but i cleaned that, or reeds could it be anything else or at least help me narrow it down

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Sounds like it is running out of fuel adjust float in carb. Possible electrical connection problem with spark from coil check all wiring make sure pet **** has flow constantly of fuel.

Posted on Oct 10, 2010

Testimonial: "helped me out. from what you are saying it sounds right from what i have experieced. thank you"

  • Kerry Matt Feb 01, 2011

    mine does the same thing what did you find

  • Kerry Matt Feb 14, 2011

    sounds like the cdi



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I have a Yamaha yz 125 2003 and there is water leaking out the bottom through a small whole and was wondering why it is doing that as it has just started doing it recently

This is telling you your oil and water seal need replacing ASAP do not ride until fixed as you will seize bottom end guaranteed !
there is small seal each side of your water pump impeller when you take the impeller out replace both even if only one needs doing as you will be going back to replace the one left soon after as they always go together..
Most people would not do this repair without replacing the impeller as well - usually the impeller starts to wear groves where it spins inside the seals then damages the seals So you get the theory behind replacing the whole lot as old impeller can damage your new seals
The seep hole is designed to give you warning of burst seal before its way to late as without the seep hole leak you would have no idea you are leaking coolant into the motor
You can do your self its a bit of work but not complicated - if you do your self don't forget to change your engine oil couple of times after first few rides reduce any risk of having coolant inside it where it should not be
good luck regards Jamie

Aug 29, 2011 | 2003 Yamaha YZ 125

3 Answers

Im 16, and ive never ridden a 2 stroke in my life and im thinking of getting a yz/sx 125 for trail riding. ive never been able to afford a modern 4 stroke so ive been riding a crf230 and a 1980 tt500 yam....

I would suggest a 250 2-stroke or a 4-stroke. Lots of power, lots of pull and the motor would likely last you for years. I've raced both the 125 and the 250 2-stroke at the pro-am level (A-class) and the difference in engine life is tremendous. I would re-ring a 125 every week (running 26:1), while with a 250 I could go for 2-3 months or more without doing the top end while running 32:1.

A YZ 125 will definately be a bit of maintenance - primarily because it has a single ring and the ring doesn't last long. A pro or semi-pro racer would re-ring every race (when the 2-stroke was popular) - some races I would re-ring in between motos. I know you won't be racing, but I would still imagine you would want to freshen up the top end at least every other month at the most. Granted - you can re-ring a 125 in about 45 minutes for a cost of about $18 if you know how and it costs a LOT more for a 4-stroke and is more of a job.

Other than the ring, the rest of the maintenance will be about the same between a 2-stroke and a 4-stroke. You'll still have to do your air filter every ride and your engine oil (tranny oil in a 2-stroke) every other ride. Tires, spokes, sprockets, clutches, chains, brakes - all pretty equal maintenance between the two. Lots of variables, of course.

The difference will be keeping the motor fresh and for that, the 4-stroke will beat the 125 2-stroke. Not the 250 2-stroke, though - these motors last as long or longer than the 4-stroke and are a LOT cheaper to maintain/rebuild.

This is MY experience and my opinion. Your mileage may vary.


Mar 25, 2011 | 2004 Yamaha YZ 125

1 Answer

Tons of smoke coming out of exhaust

A bad crank seal on the right side, if bad, will **** transmission oil into the combustion chamber. This will smoke badly.

Jul 16, 2010 | 2003 Yamaha YZ 125

1 Answer

Yz 125 2001. starting ploblem after top end rebuild

Kinda sounds like your needle valve is sticking in your carb and it is giving it to much fuel

Jun 03, 2010 | 2003 Yamaha YZ 125

1 Answer

My bike keeps fouling my plugs i got a 2003 yamaha yz 125 i chucked a br8eg plug init and a new air filter but the problem just wont go away

Two things could be at fault here. First - it's a good possibility you would benefit from a new top end. Top ends go away fast in these because they are single ringers. Under hard riding, the compression starts to fall off after just 2 hours. Poor compression means lower combustion temperatures which will lead the bike to run rich.

The other possibility is that your jetting is off. If you have had this bike for a while and it's always run well then slowly started to act up, its likely poor compression, but if you recently got this bike and do not know the history of it, then it could be a simple jetting issue - not at all uncommon for 2-strokes.

Also - no need to run the BR8EG. This is a resistor plug and is a higher performance plug. If you are a top Pro rider, you could feel the difference but for 99% of riders they could not tell the difference between a BR8EG and a B8ES. The B8ES is readily available from auto parts stores (NGK stock #2411) for $1.89 instead of $12+ for a EG plug form a bike shop. This info alone could save you a bunch of money. Keep in mind the stock plug is BR9EG, so you are running one step hotter which is fine.

Also keep in mind there is a lot of information that I do not know, nor do I know the overall condition of the bike. Internet diagnosis is always tricky, but I thought I would point these things out. I hope this has helped, but if not - we need more detailed info to properly diagnose the problem.

Feb 09, 2010 | 2004 Yamaha YZ 125

1 Answer

I have a 95 yz 125 head and 98 yz 125 bottom end.

You have a 50/50 chance that they will work okay. All the mounting holes need to line up and the cooling holes as well. Get a head gasket for the '98 and see if all lines up. Will the exhaust pipe fit the '95?

Apr 29, 2009 | Yamaha YZ 125 Motorcycles

2 Answers

No problem just done top end rebuild

Just ride it normally. Avoid super high revs when the bike is not in gear and using the power.

Please rate this solution. Thanks bookerm!

Apr 02, 2009 | 2003 Yamaha YZ 125

3 Answers

Oil is a grey colour in the engine of my yz 125 2001

Your bike is water cooled. Some of that water is leaking into the gear box. The cylinder gasket is the likely leak. I would let the shop take a look at it instead of trying to fix it yourself.

But be sure there is not an external source for the water. Do you wash the bike often? Ride the creek bed often? Leave the bike where the lawn sprinkler gets the bike wet? Leave it out in the rain? Change the oil and keep the bike dry. If the problem comes back then go see the dealer.

Mar 19, 2009 | 2003 Yamaha YZ 125

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