You could need new spark plug if your is over 2 years old. Changing the oil can help in some cases. Most of the time the problem is old gas that has gone stale. With the amount of ethanol that is todays gas this can happen in as few a 3 weeks if you are in a high humidity area. The ethanol absorbs water from the ail. Water is not good for any of the engine components. This can cause carburetor problems very quickly, it only takes 1 tank of bad gas to ruin a carburetor. Drain the gas by taking the 8/10mm bolt on the bottom of the carburetor. Get some non ethanol gas from any of the big box home improvement centers or where they sell lawn and garden equipment. This gas is usually 92 octane rating or higher. I add a carburetor cleaner to this gas at a rate that is 1 ounce to 1 gallon. That's higher than recommend but it keeps my carbs very clean. Also be sure to run the carb completely out of gas before putting it away every time. Folks will tell you that you only need to do that if you plan to not use the mower over 30 days, but I never know today when the next time I have to mow of or use a piece of gas powered equipment. Saves the cost of replacing the carb every year.