Hello darren, your description of the circumstances concerning weather and the fact that you may have old gas in the machine would certainly suggest that it could be a combination of bad fuel and possibly stuck rings from the water getting into the cylinders from the uncovered exhaust pipe. As RonG suggested, you should probably take a look at the compression, the quick check would be to put your finger over the spark plug hole and spin it over to see if it will pop your finger out of the way. After that you may want to hook up a compression tester. 120 to 130 psi. would be on the low end of what will generally allow the motor to run, 150 to somewhere around 170 psi. would be better. If you find that the compression is low, squirt some WD40 or Aerokroil penetrating oil into the cylinders and let that set for a while. Spin the motor over after letting that set with the plugs out to get rid of any excess penetrating oil and then re-check the compression. If it's still low you might need to remove the valve cover and take a look at the valves to make sure that none of them are stuck. If it appears as if one is stuck, rotate the motor over so that the cylinder position is such that the valves would be on compression, take up the slack on the rocker arm and lightly tap on the top of the rocker over the valve with a soft-face deadblow type hammer. If it isn't stuck too tightly this will jar it loose. One other note on compression, generally there is an allowance of between 15 to 20 psi. difference in compression between all of the cylinders. If there is a greater difference on a motor, that might suggest time for looking at doing some service work. Certainly change out the gas so that you're sure that you're not fighting that. Good luck and let us know how it comes out.