Gas mix, clean airway, spark, compression... It's got to be one or a combo of these. My 23 yr-old Jonsered 525 worked great and still looks good (OK - I only use it once a year). I did have to remove the carb and clean the passageways one year because it would not idle. The cleaning fixed it. I found it does like fresh gas mix. There's really not a lot to tune up... Ahhh, the simplicity of 2-cycle. Make sure the head is torqued to spec, and carb is torqued to spec. Do not worked.