First inspect the spark plug for debris and other types of gunk that may have accumulated due to weathering. If you can clear out the matter that has accumulated without much difficult and the unit looks clean enough, then it is not necessary that the spark plug be replaced with a new one. However if the cleaning process is not very effective, you may be able to go ahead and proceed to inspect the spark gap. Remember that this gap needs to be adjusted and measured from time to time to ensure that the all idling speeds can suit the engine without causing stalling. With care that you do not apply too much pressure to the low volume screws on the casing, carefully unscrew the fastening screws from the housing and take out the older spark plug unit before installing the new one. Ensure that all wires connecting the powertrain to the clutch and assembly have been disconnected before this. Once you have inserted the new unit, screw back the fastenings into place after you have reconnected all the necessary wiring.