Could be the timing gone... when you changed the plug did you check to see if you was getting a spark? when you take the plug out is the end of the plug wet with fuel or dry ? could be a numerous amount of things..
i would start with the obvious like checking for a spark... then removing carb and cleaning making sure that the correct amount of fuel is getting through... check how much oil you put in with the petrol as too much will oil up the jets in the carb aswell as your spark plug... if the plug is black or you can see fuel in the barrel and its a really dark black its possible its running to rich... to much fuel/oil to air ratio this is to do with the jetting on your car or even a dirty airfilter can affect the amount of air getting in!
If the obvious dosen't work and you have a spark then i would move on to checking the timing and things like that...
Motocross engines are very simple usually there are 1 of 2 things wrong with the either fuel or electrical... as unless there are any physical problems like holes in places there shouldn't be there are not alot of other things that can go wrong....
Hope that helped point you in a direction to start off... out of interest what age is the bike ??
Chris
March 2009
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