I have a Lawnboy mower that's 3 years old but has only been used 4 times. It was stored with old gas in it. I added fresh gas and gas treatment and was able to use it twice, several weeks ago. Now it won't start. Today, I cleaned the carburetor, bought a new spark plug, drained the gas tank and added fuel stabilizer to fresh gas. It still won't start. What should I do next?