If the engine is shut off when it is not real hot it can get moisture in it. The fuel left in the engine will suck condensation out of the air. In humid climates or humid days it is good practice to run the engine out of fuel at a high throttle setting so any leftover fuel is vaporized. Your fuel may be ok. Put a little after run oil in the engine,pull out the glow plug and spin it a little bit then reinstall the glow plug and start it. Warm it up good and you should be able to tell if the fuel is ok. Keep your fuel out of the sun and in a stable temp cool dry dark place when in storage(not the garage).