From your post it could be the carb, starting with the easiest first. Has it been rebuilt and rejetted to match the overhaul? In my experience when ever an engine is rebuilt, so is the carb and rejetted as well. Take a plug reading and start from there.

 

 

Just a carb cleaning is usually not enough, a good rebuild kit is needed with new gaskets and proper settings. If it starts good, it may be the high speed jetting that nwwds attention. Many carb kit suppliers will sell youbrhe right kit needed if you tell them what all has been done to the engine.

 

 

 

wrong jetting could definetly cause plug fouling, as well as a clogged carb, or one not properly adjusted

 

 

You need to do some testing before making adjustments. First take a compression test, then a complete carb rebuild with proper jets. The rebuild kit will give you settings for idle adjustment, float levels etc. If you tell the supplier what work has been done to the bike they will have jet recommendations for you. Also check your spark to be sure it is hot as a cold spark will cause plugs to foul also. If your not familiar with what I am talking about you need to do some research and get a manual for your machine. I can only make suggestions based on what you tell me, I cant train you to be a mechanic over this internet.