i think its your carb, have you cleaned this, are the air and idle screws ok, if so take out your plug dry it and put back in, did it kick over then and die again, if so try again with the plug , dry and try, did it kick again and die,if so your fouling your plug, the guys on here will help with that, i only know bcos i just had to rebuild a 1996 yz 80,
with same problems ,but it was down to many things, carb, crank and seals, and magnito,--- i think that if you have fuel, compresion, and spark that, one is weak, check carb , dirty, right jets?-----then your spark, spark might be duff, is there anything in the exauhst ?----- read valve look ok , strong flaps that close ok,,,
hope it might help,,, all mx pros on here dont get angry if i give wrong or bad info, im just going off my own bike