It is a yearly tradition when the last snow melts and there is wood to be processed that the equipment is due for maintenance. We purchased this bigger saw last year to process a 150+ year old cedar tree a neighbor generously gave us to use for firewood. This saw has been one of the best investments second only to the wood splitter. This year we had two 50ft oaks come down that need to be made into fire wood and this saw fits the bill (for the most part). I learned an important lesson in my youth that maintaining equipment is something you do and is needed if your things are going to last a long time.

After a stop at the local saw-shop with a fuel filter, a spark plug, and an air filter it was time to get going. After a thorough exterior cleaning and removing the chain and bar I replaced the air filter. On the newer generation of 391s the air filter is round. The first generation saws had a square filter with a handle on it. After some discussion with the guy at the counter I learned that Stihl makes two types of filter: a pleated filter in the tune-up kit and a felt style. It was explained to me that the pleated does not filter as fine of particulates. The felt style does, but clogs easier. The clogging filter I have experienced and I clean it every time I use the saw now.

To remove the air filter turn it counter-clockwise.

This will leave the carburetor extension exposed.

Replace with new filter, twisting clockwise to lock into place.

As you can see I already removed the spark plug boot to expose the plug.

The new plug for this model saw is an NGK BPMR 7A. The saw manual has the plug-gap specification. Double check the new plug as in my case the plug gapped at 0.025 instead of the 0.02 listed in the spec. I am not leaving any of that horsepower on the table.

Next I replaced the fuel filter. Wearing nitrile gloves, I used a hemostat and a narrow pair of pliers to coax out the old filter. It was slippery and requires just the right orientation to make it accessible through the fuel filler port. It took a few times and some patience to get it. The old filter just pulls off the fuel supply line and the new filter pops right back on.

The last step that I like to do is to spray out the channel in the chain bar that the chain rides in. With the amount of use this saw gets debris and oil gets caked in there. I take special care to spray out the oiling hole at the base of the bar last. This ensures debris does not clog the oiling hole after the rest of the bar is cleaned.

Now this machine is ready for the season.