[Pazuzu]

Ethanol. It's been ruining more and more small engines. It gels up as it sits.

Either drain the system after you use it (if it's going to sit for a few weeks or more), or start using something like Staybilt in your gas.

Edit: Reading your post again, I might be way off. I figured when you said "won't start after sitting a length of time" you meant between jobs, not sitting for 30 minutes. It still might be the ethanol.