Question about 2003 Yamaha YZ 125

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Hi there i have an 03 yz 125 the problem is the bike keeps fouling plugs. i run the bike up for about 10 mins till its warm then i turn off and then try to start it again and it wont. changed the plug and it will run first kick and there is oil spitting out of the back box and is very smokey plugs are going every 5 10 mins please could you help iv checked the gear oil ok and the air filter is clean and iv checked the reeds all ok. please help

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  • kyle Jul 03, 2012

    also if youve not done it yet change the packing in the back box of the exhauste
    i had this problem on my ktm 85cc



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There are many possibilities for this problem:---bad piston rings, clogged air filter, carb problems, wrong plugs.
First check the spark plug condition, when this problem occurs:-----1) If your spark plug is dry and burnt brown its normal.
2) If your spark plug is whitish it means its to lean.
3) If your spark plug it fouled with black oil over it that means your bike is running to rich. ------------- Try playing with the idle screw or air fuel mixture screw on the side of the carb.
The idle adjustments are easy...........

1.) Adjust your throttle cable free play.
2.) Start bike and warm up.
3.) Once warm, turn idle up a little high at the adjustment and then adjust low speed pilot screw to achieve the highest possible idle w/o turning any other screw.
4.) Adjust the idle screw down to an acceptable level.
------------ There should be the highest idle. That is where the pilot circuit is most efficient and gives the best off idle performance. ---------- click this link below for more help:--- ------------- This should help.thanks.helpmech.

Posted on May 24, 2011


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2003 Yz 125 starts runs good for 5 mins then Boggs down and dies won't start for 10 mins plus when kicks over runs and dies again even with feather of clutch and full throttle plZ anybody got any ideas...

The most likely thing is a faulty primary ignition coil,the one in the generator these suffer alot and when the get warm they pack up.try making the spark plug gap very small and see if it helps it. Also check to see if it has a spark just after it cuts out.

Mar 22, 2015 | Motorcycles

1 Answer


Hi, Anonymous and the usual suspects are:
1. Fuel mixture too rich or enrichner left out too long.
2. Incorrect spark plugs for your kind of service.
3. Piston rings badly worn or broken.
4. Valve guides are badly worn and/or seals worn, crushed or have come off valve guide and pump oil through it.
5. Spark plug cable weak or ignition coil faulty.
For more information about your issue and valuable "FREE" downloads that you will need please click on the links below. Good luck and have a nice day.
Why Does My Dirt Bike Keep Fouling Spark Plugs
Fouling plugs in my yz 125
YAMAHA YZ125 Owner Service Manual
OEM parts for Yamaha

Btw, I’m available to help over the phone in case u need at

Mar 18, 2013 | Yamaha YZ 125 Motorcycles

1 Answer


What is causing the fouling? Engine oil? On a two cycle dirt bike, the fouling can be caused by excessive oil in fuel mix. I believe 50 to 1 is the correct ratio yet you may want to cut back, a little at a time, on the oil or run hotter fuel. The exhaust system will also accumulate enough oil sludge to impede the exhaust flow causing poor performance and plug fouling. A two cycle must breath freely to keep itself internally clean. Air cleaner to exhaust. Proper fuel mix should have little , very little. Good Luckand hopefully this helps you.

Dec 27, 2010 | 2003 Yamaha YZ 125

1 Answer

My bike was warming up then died, has fuel in tank good flow to carb, air filter not bad, spark does look a little weak, for an 03 not many hours on it, what should i do next?

#1.) could just be a fouled spark plug.does your bike spooge alot?( leak black oily stuff from exhaust or muffler?) if so then your bike is running to rich( to much fuel to carb.)which will foul plugs often...#2.) just because you have good flow from tank to carb dont mean its getting from carb to engine you might have a parcialy pluged jet in the carb..( just remove carb take bowl off and clean with can of carb cleaner and reinstall.....If you have spark, the only thing left is fuel delivery....#3.) i left mine outside before and it rained on it and got just a little water in fuel from cap and would not run,had to drain tank and put fresh mix in and she fired right up.....should be one off these 3 things, but start with the plug first fresh mix,and if that dont do it, it will be your carb..

Sep 26, 2010 | 2003 Yamaha YZ 125

2 Answers

What spark plug should i run in my 2003 YZ 125?

Run a B9ES if you don't have a problem with fouling. If you do have a problem fouling, run a B8ES. Both of these plugs are available at auto parts stores for a few bucks each.

The stock plug was a BR9** - the R indicates a resistor plug which is not needed.


Jun 18, 2010 | 2003 Yamaha YZ 125

2 Answers

2000 yz125 keeps on fouling out plugs

Fouled plugs are typically caused by jetting that is too rich. This could also be from a dirty air filter which can make the bike run rich. Clean up your filter and see if that is it. If the filter is not too bad, you might want to drop a size on your pilot or main.

Another possibility is poor compression - this can make a 2-stroke run richer than normal as the fuel can not be burned properly. Rings don't last long in these 125s. I used to re-ring each week when I was racing 125s.

You might want to run a B8ES plug, too - this is one step hotter and will help the plug from fouling as often.

Feb 01, 2010 | 2004 Yamaha YZ 125

2 Answers

2003 Yamaha 125 yz plug fouls pretty quickly. running 32/1 fuel oil ratio, and using NKG plug 2411/B8ES. Do we need to run a hotter plug?

NGK BR9EG is the stock plug for your bike. You are one heat range hotter already. I suggest you go to a BR8EG to keep you in the racing plug style but one range hotter than stock. I question what 2 stroke engine oil you are using. Stick with Yamalube 2 stroke oil and you should be okay. If you are using standard motor oil like you have in the gearbox then the fouling is definitely the oil.

Oct 27, 2009 | 2003 Yamaha YZ 125

2 Answers

Oil and gas mixture coming out by exhaust

first off make sure the oil gas mixture is correct and mixed very well.... have the carb cleaned out very well or even rebuilt, jetted and adjusted...

Jul 06, 2009 | 2003 Yamaha YZ 125

1 Answer

Yz 125cc 2003

Black spark plug means too rich , you will have to replace the spark plug since it has foul up and reset the air mixture screw on the carburator 2 and 1/2 turns outwards.
Check the air filter too.
Hope this helps.

May 23, 2009 | 2003 Yamaha YZ 125

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