If the air filter is okay, the next step is to check the fuel filter. If the saw runs with the filter removed, that is a dead give-away that it is the source of the problem. Remove the filter by giving it a twist to the left. If it is starting to deteriorate, replace the air filter. Make sure it is clear of debris and vacuum it out if necessary. Use a large flat screwdriver to remove the 3 screws from the top cover of the saw. The next thing to check is the air filter. With fresh gas/oil in the tank, give the saw another try. Stihl website says that gas with up to 10% ethanol is okay. Find a local dealer to get it from, as it is usually cheaper that way than buying it off the internet. I recommend using the Stihl 2-cycle oil as well. That’s 2.6 oz of oil per 1 gallon of gas. The correct ratio of gas to 2-cycle oil is 50:1. Shake the saw around to get as much out as possible. If this is the case, pour out the bad gas into an approved container. Stihl recommends not leaving gas in the unit for more than 60 days. 95% of all chainsaw ‘problems’ are a result of ‘bad gas’ which usually means that the gas was left in the unit for several months or more. We’ll start with the most common problem by far when it comes to small engines not starting.
![stihl check size of cylinder stihl check size of cylinder](https://cdn3.volusion.com/dafjy.dwcca/v/vspfiles/photos/CKMS311-49MMBB-2.jpg)
#STIHL CHECK SIZE OF CYLINDER HOW TO#
![stihl check size of cylinder stihl check size of cylinder](https://i.ebayimg.com/images/g/~NgAAOxyuOtRZoK~/s-l300.jpg)
The cause of the scored piston and cylinder was two-fold: incorrect oil/gas ratio, as well as the saw being set to run too lean. Ultimately the problem was due to low compression.
![stihl check size of cylinder stihl check size of cylinder](https://m.media-amazon.com/images/I/61g0e8qRRFL._AC_SL1000_.jpg)
Here I document, with pictures, the procedure taken to repair a Stihl MS291 Chainsaw that would not start.