Yz 125 fouls spark plug when it bogs. - YZ 125 Yamaha Motorcycles

SOURCE: bike bogged down like plug fouled now wont start

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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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

SOURCE: my yamaha yz 125 is dumping to much fuel and

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.

SOURCE: 2003 Yamaha 125 yz plug fouls pretty quickly.

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

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.