Maximum time allowed between engine run up

Hi Ray

The detailed information is found in the Heavy Maintenance Manual.  The recommendation for new and then engine in service is given.  It has the process but in the 912/914 manual does not have recommended times.  They have some new info on that in the injected manuals and will update the carb ones at some point.  

For your engines it is imperative, as pointed out by others above, to drain out the fuel.  Fuel becomes stale and running on old fuel has a big risk of putting the engine into detonation if degraded too far.  If it is a long term layup then it is best to do an oil change just before putting it into storage.  We want to drain out any contaminants that might have built up in the oil before storage.  Given you are in a humid area by your notes I would also be very sure to seal up the exhaust and the intake to avoid any moist air entering the engine over time.  This is a good idea even for a relatively short period of say a week.  Engines breath with temperature.  Warm engines, say parked in the sun, when they cool down at night will draw in the moist night air, this can result in corrosion on items like the valve seats and exhaust guides or the piston rings.  if there is any ethanol in the fuel over time it will also absorb water and at some point this is a concern if you have enough build-up in the system.  

Consider, a big benefit of proper avgas, 100LL or Swift Fuel 94UL: stability. Year-old avgas is still legal for sale and will meet spec for another couple of years after that. Unlike mogas, it does not form gum or varnish, nor does it depend on light aromatics to increase octane, ethanol or not. Avgas also evaporates "cleanly" by comparison. Draining mogas before winter storage I'd call mandatory, but in many cases you won't get it all out. "Unusable" remains will do bad things. If you have the option, think about running avgas for the last couple of tanks before season's end. Drain if you feel like it, but letting avgas sit for a few months over the winter is no problem whatsoever.

With respect to water entering fuel through water vapor in the air, the EPA says this:

Water, in the form of water vapor, can dissolve in gasoline.  The more humid the air, the faster the water vapor will dissolve in the gasoline.  Due to chemical equilibrium, however, assuming a constant temperature, phase separation will never occur if the only source of water is from the air.  Only enough water to saturate the fuel can enter the system, and no more.  Water vapor, however, dissolves in gasoline very slowly, even at very high humidity.  For example, at a constant temperature of 100 degrees F and relative humidity of 100%, it would take well over 200 days to saturate one gallon of gasoline in an open gasoline can (assuming the only source of water is water vapor from the air).  Water absorption from the air is far slower at lower temperatures and humidities.  At a temperature of 70 degrees and relative humidity of 70%, it would take over two years to saturate one gallon of conventional gasoline in the same gasoline can.  Oxygenated gasoline (i.e. E10) can hold much more water than conventional gasoline, and would therefore take much longer to saturate with water.