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Here's the way I've heard of it happening (this is my first actual experience with this problem): 1. Fuel system is gravity feed so even when the tractor is off the "fuel pump", ie, gravity is still operating. 2. Two things are in place to prevent gravity from draining the tank. One is the shut off valve on the sediment bowl. That one is dependent on the human touch. If no one shuts it off, it's still on. The other is the float valve in the carburetor. If it stick open or gets a piece of crud stuck in there preventing it from closing fully, then gas is gonna continue to flow. 3. If the tank is full enough, and the two valves mentioned in item 2 are both open, fuel will flow until the carb is full. Then it will migrate up past the open throttle butterfly into the manifold. At least one engine valve is sure to be open and so the gas will flow into the cylinders where there are valves open. 4. Finally the fuel seeps past the piston rings into the crankcase. Believe me, the smell coming off that dipstick is unmistakeable. OLD GAS!! Another horror story I've heard from the leaky gas valves scenario is that the gas fills up a cylinder but before it can drain into the crankcase someone comes along and tries to start the engine. As the piston comes up on the compression stroke in the cylinder full of gas, it locks up because the gas cannot be compressed the way the air/fuel mixture can. I've heard of people bending con rods and other bad damage from a hydro-lock situation. So, I always shut off the gas when I'm not using my tractor since I can't always trust the cheapy float valve in the carb.
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