How often should I replace the spark plug and air filter on my chainsaw?

Replace the spark plug once a year or every 50 hours of run time, whichever comes first. Check the air filter every few tanks of gas and swap it when it’s visibly grimy or clogged. In dry, dusty conditions, clean or replace the air filter after every use; for occasional firewood, you can go a whole season. Don’t wait until the saw bogs down—by then you’ve already been running dirty.