Canoedogs,

I've also left gas in my lawnmower over the winter and it still ran (unfortunately ). I think small engies like these can run on much dirtier gas than a car would need.

There are two problems with old gas:
1. Loss of volatile components which results in the gasoline having a higher ignition temperature. A hotter-firing sparkplug may help there.
2. Polymerization of heavier molecules into ultra-heavy components (varnishes and gums, think paint drying) which clog up fuel injectors, valves, and other components.

You can run your car/truck on old gas, but after a while you'll gum up your fuel system something aweful. I don't know if any of the current fuel additives help much with cleaning out this varnish.

-Blast