it will start and rev up easily enough but after 20-30 seconds it dies.this saw ran fine a year ago. its a friends saw and he was using it up until last summer. he told me just to take it and fix it. i could have it if i wanted it, he thinks it's the crank seales. he already checked/replaced the fuel line cleaned and tuned the carb checked the boot and gaskets pulled the cylinder, compression test, spark... no problems that he could find and he would know if he found one. its a well maintained and fairely clean machine. im not ruling out any of the things listed but where do i start