it could be one of the FF:
One of your sparkplug is dead,
One of your hi-tension wire is damaged (by rats?)
You have droplets of water in your Fuel
Or WORST SCENARIO;
You have overheated your Engine and the Pistons & Liner got damaged heavily... Better get a certified mechanic and have it check. Or check the splugs, hitension wire, fuel(chk the fuel filter if it has small white bubbles then there's a POSSIBILITY of water...) Good Luck!