sounds like your carb needs adjusting or re jetting thats the best thing to do if your carb is running to rich that will cause it to foul the plug
that should fix your problem

 

I personally like the FMF jets sold in the denidkerk magazine and its simple to do you half to take your plastics off and then your gas tank to get to the carb then once you have changed them gust set your gas tank back on DONT bolt anything back down yet untill you have everything right you will be able to tell when its right