Question about Yamaha YZ 125 Motorcycles
Posted by Anonymous on
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.
Fouling plugs in my yz 125
Yamaha YZ125N1 Service Manual
OEM parts for Yamaha
Posted on Jul 14, 2016
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Posted on Jan 02, 2017
Based on the information given, I think you may have a hole in the top of the piston. It sounds like pre-ignition, ( AKA detonation and engine knock ). Pull the cylinder head and check the piston and cylinder head. Sometimes the top outer edge of the piston disintegrates due to the heat. Replace the piston and rings and remove the carbon from the head. Be sure you run 32/1 mix on gas and oil. That's 4oz two stroke oil per gallon of gas. Only use premium gas.
Remove and clean the carb. Be sure the carb mount bolts and carb manifold bolts are tight. An air leak will cause the engine to run hot.
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Posted on Apr 07, 2009
SOURCE: what spark plug for yz 125?
I have a dennis kirk book, the correct spark plug is a --BR9ES--(NGK)
i always used SAE 10w-30 oil on all my dirtbikes,
no transmission fluid
for the chain you should move the back tire back until the chain is at a snug tension
Posted on Apr 24, 2009
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.
Posted on Nov 02, 2009
Like he said BR8EG runs hotter for yamahas and is bettter because br9eg foul quickly. Use castro in 2 stroke bikes. It runs a little higher because its better
Posted on Nov 17, 2009
SOURCE: yz 125 bog
Sounds too lean to me or lacking fuel.
Partially blocked jet in carb. May have to remove / clean.
Check also that carb butterfly / piston is travelling to full open position.
Posted on Dec 21, 2009
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