2001 YZ 125 Fouling sparkplugs What to check first?

Answer 1

Fuel oil too rich. Jetted too rich. Run with choke on too long. Make sure choke is off after you put the new spark plug in it. (Choke it, start it, warm it up, take the choke off, ride it) If it fouls still, recheck that your fuel oil ratio is not too rich, then thirdly if another new plug still fouls then jetting will need to be changed. But beware, too lean and you will not be buying spark plugs but you will be buying pistons... If it fouls a plug every now and then, months, I wouldn't fret too much. If it's happening every weekend, figure it out. And, the colder the temp outside, the LEANER the air fuel mixture will be, I.E. rejetting will need to be done so again, be careful leaning things out too much too quick!