My kx 125 only works for 45 mins then have to put new spark plug in if want to start it again any ideas?

what kind of oil are you mixing in gas and at what ratio? if you are running a synthetic, you should be at about 32:1 - 40:1. if you are using cheap non synthetic, then 20:1-28:1. it is fouling out your spark plug because you are either A- not mixing the fuel oil correctly, or B- maybe not riding it. it will foul out the plug if it sits and idles alot. as long as you arte riding it and riding it on the powerband, then i would say it is the type and mixture of the oil.