MY 1998 YZ 125 KEEPS FOULING PLUG AND THEN BOGS DOWN

SOURCE: spark keeps fouling but no clue why

Are you losing gear oil.
When the crankshaft seal leaks, oil from the gearbox is sucked into the crankcase, and this can foul the plug.
Also check the plug gap, most guys set the gap too small. It should be 0,7mmto 0,8mm. The plug should be a B9 EG, or equivilant.
Now, you can also change the plug cap to a type that has a built in resistor, this also helps with plug life.
Note that 2 strokes use plugs way more than 4 strokes, and you should replace the plug every 15-20 hours to keep your riding pleasant.
Good Bike you have there
Cheers
Jon

SOURCE: yamaha yz 125 2001.water leak

Yes, the water seal is leaking. This is the indicator hole for the water and oil seal.
Enjoy the ride !

Jon

SOURCE: Fouling plugs

Get a new NGK spark plug one heat range hotter than stock. Be sure the fuel mix is 32/1 using two stroke engine oil ( not motor oil ). A 32/1 mixture equals 4 ounces oil to 1 gallon of gas. Remove the cylinder head and de-carbon the top of the piston and head.

Please rate this solution. Thanks!

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.