I acquired a Craftsman FS5500 with the Intek 24HP V-twin motor. It hadn't run for 2-3 years and after cleaning it up, topping off the oil and putting new plugs, filters and fresh gas (and some fuel system cleaner) in I started it up and it runs okay but seems to have a vibration at certain RPMs. Not enough to shake my teeth out but my Husqvarna with the same basic engine doesn't do it. The output shaft is rock steady when it's running so I'm wondering if it could be an ignition issue. The plug on the left side seemed to be a little blacker than the right side one. Has anyone else had this happen and what could be causing it? According to the gauge the motor only has about 98 hours on it.