I have a echo 260t that last year started running rough and half powered. I have the carb adjusted by me and a echo dealer which made it run alittle better. The spark arrester has been removed, exhaust port cleaned and muffler checked but would still go back to running poor. yesterday I stuck a new muffler on it and it still wants to do the same thing and run half powered and bogs down when i try to edge with it. I alm running opti oil @ 50:1 mix and using unleaded(non ethanol) gas. I also replace the fuel filter, lines, air filter and spark plug. Any ideas?