As the engine runs, the float inside the carburetor drops opens the float valve, and gas comes pouring in. Fuel flows in, the float rises, and the valve closes. If the valve cannot close, gas keeps flowing, even when the engine is off. This gas can fill the carburetor and drip into the cylinder/s, seep past the rings, and end up in the crankcase with the oil. A little mixing is okay; but oil & gasoline are supposed to be on opposite sides of each piston.
1. The needle and seat are not sealing and fuel is running into the engine and PCV tube filling the crankcase. This can be caused by a worn needle and/or seat or dirt under the inlet needle. It can also be caused by the float having a leak or the pivot pin not allowing it to move freely. You can visually inspect the inlet needle, if it has a groove worn in it, it will need to be replaced. If this doesn’t correct it, you may need to replace the seat as well and I recommend you have a shop do this for you. If you find dirt under the inlet needle, you should clean the entire fuel system and replace the filter. You should be able to shake the float and listen if it contains liquid; if so, you will need to replace it. If the float doesn’t move freely on the pivot pin, determine what is causing the problem and correct.
You can replace the float inlet and seat, which is normally an inexpensive part or you can install a fuel shut-off valve for when the engine is not running. Make sure you clean the entire fuel system including the tank, filter and lines.
2. If your engine has a fuel pump (as opposed to gravity fed fuel), it is possible the diaphragm has a hole that is allowing fuel to run into the crankcase via the impulse passage (crankcase pulses operate the fuel pump). Be sure to completely drain the crankcase and replace with fresh oil.
Other possible causes could be that the plug in 1 cylinder is not firing, and the gas is not getting burnt and is then mixing with the oil in that piston chamber. The engine will run rough, and the power will seem a lot less than normal.
The piston rings in one of the piston chambers may also be cracked, and you are getting the oil and gas mixing in the chamber and it’s not burning all the gas when the plug fires. This would let a lot of gas go out into the oil reservoir and cause this problem.
If the engine stopped running and you can?t restart the next day, you need to pull the spark plugs and take a look at them and see if any of them are coated in oil. Clean the plugs and see if it runs.
If not, then you may have a more serious problem, like the engine block may have a crack. If the engine block is not cracked, most likely the piston rings are worn and are allowing gasoline to seep past the piston into the crankcase.
Engine blocks can get cracked from many different activities:
- hitting an obstruction with the blade
- low or high oil levels putting excessive stress on the block
- thermal shock, or washing down a hot engine with cold water