change the plugs, wipe around the plug leads and caps.
clean / change the air filter.
if it is a carburetor type drain the carbs to get any cr4p out of the float bowl. also check that there is no water sitting in the bottom of the tank, petrol floats don't forget. (drain it totally dry then refill. )
is the fuel filter ok ? or could it do with renewing?
make sure the inlet stubs are ok and not cracked letting air in.
lastly, take all the connector blocks apart and WD40 them one at a time ( look in the blocks before rejoining them to see if there's any corrosion. )