1999 ktm 250 exc pinging

was wondering if anybody can help with the pinging on my bike. It seems to only do it when the bike is almost at idle. I use premium fuel , have a 175 mj a 45 ij and have messed with the clip position on the needle and get the same result. It seems to only do it when the bike gets hot. Took the cylinder head off and found that there is some carbon build up on the piston. could this be the problem? Thanks Tom

Pinging

Hi, was wondering if anybody can help me find out why I am having pre-detonation on my 99 KTM 250 exc. Previously I adjusted my jetting. I have a 145 MJ, 45 IJ and have went from top to bottom with the clip on my needle and have the same symptoms. recently I just took off the cylinder head and have found some carbon build up on the top of the piston. Doers this get hot enough to have pre-ignition?

Any other suggestions would be greatly appreciated. Thanks Tom

Thanks for your help. Cleaned the build up off. wont know for sure if I fixed the problem till next week. I tried mixing race fuel still no luck. Thanks again Tom