Again, check the ignition. Check the output at the spark plug end of the hi tension wires. The spark should be bluish white, if it is somewhat orange, the spark intensity is weak and can not provide sufficient voltage to ignite the air/fuel mixture with increasing heat, pressure and lean air/fuel mixture. Check this first. An evidence of weak spark is carbon fouling of the plugs. Dalhin mo sa tuli na mekaniko o sa trained technician