Dump out the mix in the tank and try fresh mix.

Probably need to clean the clutch. It won't rev up with the brake on cause the brake is holding the clutch drum when enough rpm over takes spring pressure on the internal two halfs of the clutch it want to try to turn the chain by centrifugal force acting on the outer drum and bog the engine down. try starting it without the throttle locked in full throttle with the brake on. it should start and idle if not you probably got clutch problems

Thanks rcmoser. Took saw apart - clutch was caked with packed sawdust and oil. Cleaned it all up. Chainsaw now runs with brake on, but doesn't like to idle very well. Working on that.