Lawn Mower sputters

hello! We have a Toro personal pace lawn mower that has given us many years of good service. A couple of weeks ago, however, it started to run a little weaker. Not knowing a lot about lawn mowers, I changed the air filter thinking that might be the problem. But it has progressively gotten worse. Now when it starts, it sputters really bad and when you start mowing it almost always shuts off like it doesn't have enough power to cut grass at all. I have thought that it could also possibly be the spark plug but I don't know. We are planning on taking it in to get it looked at by a professional, but that always costs money and doing it yourself is more fun and cheaper, especially when it turns out to be something simple. Any thoughts and suggestions would be helpful.

 

What egine does it have?
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Well, just as I said, something simple. The one part we forgot to look at after everything was the problem. We checked the carburetor and it was full of grass. Cleaned that out and it worked just like it did before.