quits when hot

[bubba107]

hi my kc will run fine for about 20,30 min then dies and will not start again unless i put in another spark plug , replaced the carb plugs arnt fouling anymore tips are coming up grey white. any ideas , i cleaned up the points,there was a dark film on them . the point contacts are a grey colour . should i sand them to a shine or is grey normal .didnt look pited. am i going in the right direction or should i be looking at somthin else?