Fouling plugs 1998 KX 125

I would use a NGK Platinum Plug (R6918B-8 or BR8EIX/BR9EIX) and try to cut back a little on the oil mixture to gas ratio or adjust the gap a little hotter.

If the plugs are fouling from excessive oil then it may be that the oil ring is broken. That will be scoring the bore and accounting for the low compression pressure. To do compression checks the motor has to be hot and the throttle wide open. Check for blocked filter before testing. IN the short term run a hot plug . ( where the porcelain insulator is very long in the plug) as this will help it burning off the oil .

Is it possible that rings were broken during the assembly?

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.

if your fouling plugs then either your fuel/mix ratio is off ( should be mixing 32:1 ) (or) your bike is running to rich( getting to much fuel to engine) which means you need to rejet the carb....example:( if you have a #50 jet,you need a #48 or 49) to install a new jet, #1.)remove carb,#2.)remove the bowl(4-screws),you will see the jet and #3.)unscrew jet and replace with smaller # ( number is stamped on the jet.) you can get jets from local motorcycle dealer( but they will probably have to order them). or go online to bikebandit.com enter your bikes year,make,model,ect.and just order from them........

if your still fouling plugs then you still need to go with smaller # on main jet...are you spooging allot out of pipe and muffler? then your to rich still,smaller jet size!!!! 32:1 is a good mix ratio

32 to 1 is the standard mix. That is 4 ounces two stroke ENGINE oil per gallon. The brand doesn't really matter. Don't use MOTOR oil in the gas. The stock plug is NGK brand, R6918B-9. I would suggest a hotter plug, R7376-8. The last digit is the heat range. The lower the number, the hotter the plug.