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What To Do If You Find Gas In Your Oil, In Your Riding Mower Engine

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

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2 Answers

I have a 2009 super bronco riding mower won't start, but when I put gas in the carb with a straw it runs. Does this 20 horse Koehler have a solenoid?

it could be a bad fuel pump, dirty float bowl causing the float to freeze shut, not allowing fuel into the carburetor or the electric actuator, on the bottom of the float bowl, is frozen shut from corrosion or bad fuel, also not allowing fuel into the carburetor. also check the fuel filter.

Mar 23, 2015 | Garden

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why does gas pour out of my harley davidson mikuni carburator?

you probably let your bike sit for a while and your float is stuck in the carburetor,so try tapping lightly on the carburetor bowl at the bottom to see if it stops flowing .anyway it goes you probably looking at taking carburetor off and cleaning it out

Dec 21, 2014 | Harley Davidson Motorcycles

1 Answer

gas leak out or the carburetor

Thiis usually caused by a failure of either the float valve or the float itself.

Bad float needle valve: If some dirt or other particle gets stuck on the needle valve or seat, the valve will not be able to completely close and fuel will continue to flow into the bowl and eventually overflow. A new needle valve and seat are < $5.00 on-line.

Bad float: If the circular float gets a hole in it, fuel will get inside and the "float" will get heavy and no longer float on top of the fuel in the bowl. Because it doesn't float it will not rise high enough to push the needle up into the seat and keep extra fuel from entering the bowl. A new circular float is < $10 on-line.

Aug 28, 2012 | Coleman Powermate Portable Generator...

1 Answer

gas is dumping out of my carb from all seams 1986 w100 dodge 318

If you have gas coming out of the top of the carburetor then you could have either a bad float, a stuck float or a bad needle and seat, all of which are inside the carburetor. The float rises with the level of fuel and shuts off the incoming fuel by pushing the needle valve against the seat thus stopping any more fuel from entering the carburetor until the fuel level drops slightly at which time the needle moves slightly away from the seat and allows more fuel to enter again. You could also have a piece of crud that has gotten between the needle and seat which will prevent the valve from fully closing but that's pretty rare. You will probably need to have the carburetor cleaned, inspected, and have a repair kit installed to fix the problem.

Jun 08, 2010 | 1986 Dodge Pickup 4WD

1 Answer

dies out like it ran out of gas after 15 min. but will restart after it sits for a while

The fuel is not getting into the carburetor as it should, the issue may be one of the following:
  1. Clogged or restricted fuel tank exit.
  2. Clogged or restricted fuel filter
  3. Fuel shut-off valve not open fully
  4. Float valve in carburetor maladjusted, fouled
  5. Excessive water in float bowl

This is what I do to check 1,2 and three all at once.
  1. I run the mower out of fuel after draining as much as possible into a suitable container.
  2. I remove the fuel line from the carburetor and direct it into a catch container such as a clean water or soda bottle.
  3. I add a small amount of fuel to the tank (less then the catch container you are using) and observe how quickly the gas flows from the tank to the catch container. If the fuel trickles in the issue is likely one of the three described above. If the fuel drains into the catch container quickly then the issue is internal to the carburetor which may require a trip to the local small engine shop.
  4. Once all of the fuel has drained into the catch container re-attach the fuel line to the carburetor. If the mower is equipped with a push button drain on the bottom of the carburetor press it momentarily to ensure that the float bowl is not partially full of water (common in the spring). If more than a drop or two of water came out you may have solved the problem.
  5. Place a folded shop towel or paper towel under the drain button to hold any fuel. I recommend wearing nitrile exam gloves for the next part, pour the fuel from the catch container into the fuel tank while pressing and holding the drain button on the bottom of the carburetor, under normal circumstances with items 1,2 and three testing out ok the fuel will run out of the drain button in a steady stream, if it comes out in drops then the cause is likely the needle valve or its related circuitry within the carburetor, this is best handled by those with advanced experience such as a small engine repair shop.

May 12, 2009 | Craftsman Garden

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