Read the plugs, remove them... they should all look dry and clean with a brown or whitish tint, if they are oily or wet looking or have carbon build up replace them, usually plugs or spark plug wires are the simplest cure unless you have major mechanical problems such as a worn engine or bad valves also you might check to see if there is water in the gas, put alcohol in the tank if you suspect water, rubbing alcohol works fine or any additive to remove water from your tank.
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Read the plugs, remove them... they should all look dry and clean with a brown or whitish tint, if they are oily or wet looking or have carbon build up replace them, usually plugs or spark plug wires are the simplest cure unless you have major mechanical problems such as a worn engine or bad valves also you might check to see if there is water in the gas, put alcohol in the tank if you suspect water, rubbing alcohol works fine or any additive to remove water from your tank.