Question about Yamaha YZ 125 Motorcycles

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1987 yz 125 no spark not starting

Road bike test had put 330 main jet in it road it for about 15.min hard through the higher gears as we do flat track, and noticed it started to break down at the higher reving range then seem to cut in and out on the way home. then would not start at all then i changed plug no differance also checked plug for spark and there was no spark, i think maybe coil as it looks fairly old or maybe stater.

Posted by Anonymous on

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  • Yamaha Master
  • 39,932 Answers

Hi, Anonymous and the usual suspects are:
1. Fouled spark plugs.
2. Severely discharged or a damaged battery, should have 12.5 volts or better and be able to pass a load test if necessary.
3. Check battery terminals for damage or corrosion, check the battery cables at "BOTH" ends for loose, corroded, or broken connectors, "INSIDE" and outside the cable harness, perform connector wiggle test and check cables with an ohmmeter if necessary.
4. Loose connection at ignition coil or plug between ignition sensor and module.
5. Spark plug cables in bad condition, shorting / leaking, spark plug cable connections loose.
6. Faulty ignition coil or electronic control module or CDI box.
7. Faulty CKP, CMP, or BAS sensor.
8. Faulty ignition switch.
9. Tilt sensor needs a reset.
10. Security alarm failing to disarm needs reset
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.
Dirt Rider Magazine
How to fix No Spark Problem on Motorcycles and Dirt Bikes
YAMAHA YZ125 Owner Service Manual
OEM parts for Yamaha
http://mybikemanuals.com/yamaha/yamaha-yz-owners-manuals

Posted on Jul 18, 2016

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Posted on Jan 02, 2017

  • 3567 Answers

SOURCE: hard start, trouble idleing

Drain the carburetor. There should be a screw on the lower side of the carb float bowl. Remove the screw then replace it after the fuel drains. Turn the gas back on and wait a minute for the carb to fill with gas. Install new stock spark plugs. If the bike doesn't start and run properly then shut off the gas and remove the carburetor from the engine.

Remove the float bowl and clean the entire carb with a spray carb cleaner from the auto parts store. Wear protective goggles to avoid getting spray in your eyes. Spray into all the little airways and fittings in the carb. This process should get you back on the road.


Please rate this solution. Thanks!

Posted on Apr 02, 2009

  • 3567 Answers

SOURCE: bike only fires once

Okay, let's have a look at it. Say Ahaaa.
The probable cause of the problem is not the spark. Get a fresh set of Stock NGK spark plugs, ground the plus to the engine, and check each cylinder for spark as the starter button is pressed for 3 seconds or so. I will wager that the spark is present.

The likely problem is the fresh gas was put into a very rusty gas tank. Bits of rust now inhabit your carb and are having fun plugging up the carb. Look at the gas valve. With a little luck there is a small bowl behind the valve. Remove it and tell me what you see in the bowl.

Posted on Apr 11, 2009

SOURCE: 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.

Posted on May 23, 2009

  • 440 Answers

SOURCE: I have a 2005 yamaha venture royal star and i just

If it wont do anything then it is likely that youve popped a fuse
But it sounds like the battery isnt being charged properly. Make sure the voltage across the batt terminals is at least 13.5v at tickover rising to no more than 15.5v at higher revs
Ride-Safe
Bike-Doc

Please mark this as helpful (if it has been!!)

Posted on Aug 14, 2009

Testimonial: "thanks, I will try what you reccommended"

  • 1 Answer

SOURCE: engine turns over but wont start

could that be a possible fuel line crack? or the pet **** is fouled up. i got the same problem found 1/2 in crack in the fuel line. from trying to start it before i found the crack i dryed out the carbs. im a girl and its my first bike just learning

Posted on Oct 15, 2009

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1 Answer

My yz 125 started to bogg down when i put my bike in 3rd gear and i drove it back the next day i went to wash it and i tried to start it. it started but bogged right down till it shut off no matter where...


Ii sounds like your bike cuts out when it get`s hot ? If it does this like clockwork then i guess you have same problem as me with my no limit 125 which does exactly that. i have fixed all connectors & cdi but it still does it. Which leads me to believe some ferrite core in the ignition system is going non-magnetic (called the Currie temp) at this "hot" temperature. It can either be the magnets in the flywheel or the core of ignition coil.I only use this bike for trips less than 10km so i live with it. if anybody can shed more lite plz!

Aug 05, 2013 | 2005 Yamaha yZ 125

1 Answer

Kx 125 problems


it runs but sputters.
that can be 2 things.
First idd your sparkplug cap has had it (get a new 1 from the parts store)
2nd it could be that your getting to much fuel in top end of your revband.
easy to check just run your bike a few min with a new sparkplug and look at the color.
Wite is not enough fuel.
Brown is perfect mix.
black is to much oil and if black and wet its to much fuel.
try this test and maybe you'll find it

Apr 01, 2013 | 2003 kawasaki KX 125

2 Answers

How do you set the timing on a 01 yamaha yz 125 or why else would it be melting the spark plug prob off with a 32:1 fuel mix and the plug that goes in the bike from the factory


melting spark plug is too lean. exhaust gas high temps caused by too much air. your oil to fuel ratio is correct, but your air to fuel ratio is not. adjust your needle clip position to add more fuel to the carb by lowering the C clip. this will raise the needle, giving you more fuel for the same amount of air. also check air filter element for tears.

Nov 22, 2010 | 2003 Yamaha YZ 125

1 Answer

Yz 125 wont start


gunk in carb jets pluged

Jun 18, 2010 | 2003 Yamaha YZ 125

1 Answer

Need spects for carburetor main jet on mikuni ucal and spects for pilot air jet 1 and pilot jet 2


http://www.cmsnl.com/yamaha-yz125r-2003_model11245/partslist/A-12.html

Site above has all the parts and specs for your bike. Jet ranges are large for a YZ as many mod's are available and riding conditions contribute largely to a peaky 2 banger. Start with #40 pilot, and #420 main. Trial and error from there. Cheers.

Apr 26, 2010 | 2003 Yamaha YZ 125

1 Answer

My yamaha yz 125 is dumping to much fuel and fouling out the plugs but i need to knoew how to adjust my carb its a 1997


There are only two adjustments to make on your carburetor; the idle speed, and the idle mixture adjustment. If the spark plug fouls frequently, first make sure you are using the correct spark plug. Your 1997 YZ125 uses an NGKB9EGV. Then be sure you are using the recommended two stroke oil, mixed to the proper ratio, in the fuel. Also make sure the air filter is not excessively soiled.
With all of the above confirmed and corrected, start the bike and run until the engine is up to normal temperature. Adjust the idle mixture screw to attain the fastest idle and best throttle response. Then adjust the idle speed.
If you are still fouling the spark plug, verify that the carburetor has the correct size jets by consulting your owner's manual. Keep in mind that if you are operating your bike at an altitude higher than about 6,000 feet above sea level, the stock jetting may be too rich. Down-size the main jet in small increments until satisfactory operation and the correct spark plug color is obtained.

Nov 02, 2009 | Yamaha YZ 125 Motorcycles

1 Answer

How do you know you should go with a bigger main jet? What exactly would it do for my 2001 YZ 125?


a bigger main jet will make the bike run richer which means more gas.
unless you have changed something on the bike which would involve a bigger main jet like porting or something like that. I wouldn't mess with the jetting.
if you are trying to get the bike tuned properly, there is an adjustment screw on the carb, but I would put it in the hands of a pro.
You can easily blow your bike up messing with jetting.

Jul 21, 2009 | Yamaha YZ 125 Motorcycles

1 Answer

Hello, I have a YZ 80 and i was wondering why my


Look at ?

crank seal (drawing air)

ring wear

main jet (blocked jet's)

fuel starved carb

Jul 15, 2009 | 2003 Yamaha YZ 125

2 Answers

Bike not starting


Its easy to test. take your plug out and ground it then crank your motor and check for spark. If no spark then its your ignition. probably a bad capacitor. If its your fuel system, drain it then, clean it then refuel.

Mar 15, 2009 | 2004 Yamaha YZ 125

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