I have a Stihl 031 that someone gave me and had old gas in it. The saw has not been started in 9 years. I drained old gas out and put new in. Would not fire. I took spark-plug out and poured small amount of new gas in and replaced spark-plug and then it fired right up and would run until gas used up. What do you suggest that I do to get it to run? Thanks in advance for any help or advice.