Oil, spark plug, gas. Those are your 3 major components of the engine. I knew nothing about lawnmower engines until I had issues with my cheap one in Wisconsin. I bought it at Wal-mart (first mistake) and after using it a year, it started over-carburating itself. I had a buddy who knew lots about engines that couldn't figure it out. So I left it when we moved to Texas. Then my dad gave me his 18 year old self-propelled mower that the self-propelled part didn't work, so he got it fixed. After I got it and started using it, the self-propelled part didn't work, so I was pushing around a really heavy mower that wouldn't propel itself. So now I have my father-in-law's old mower, which works and propels itself much better!