Thats pre-ignition according to your description. Now check mo timing mo baka naka advance sobra. The simplest way to know if advance or retard is try starting it, pag hirap umikot makina then mali timing mo, hence pre-ignition. check also adjustment of your carburator, baka masyado naka bukas ito kaya even if you switch of the engine eh may gasolina pa in the cylinder head kaya it still keeps it running for awhile. Contact point clearance has to be exact din. If symptoms persist call a doctor........ hehehe.....