In case you didn't read the comment thread -
- Ethanol-free gas (100% gasoline) for power tools is now available in some Home Depots
- For smaller trees and stumps, cut them to ground level with a reciprocating saw. I have a cordless Sawzall (wonderful tool), but the thought never occurred to me to do that. Chain saws can never touch dirt, but a Sawzall don't care.