I was wondering if any of you had an idea what might be wrong with my lawn mower. It's 7 years old, some model I bought at Lowe's, with a 3.5hp Briggs&Stratton pull-start motor. Last summer it started having a strange idling problem, where it would start, the revs would rise to normal and then they would drop to a low idle just above stalling, then back up to normal, down again, and so on. Kind of like a throb. I've changed the oil and air filter and spark plug, but that didn't affect anything. Could the carbeurator be fouled, or the fuel filter, or fuel tank be a problem? I'm just wondering if there is any simple fixes to try before I just give it away and buy a new one. Thanks for the help.