How a Small Engine Crankcase Breather Works and How to Quick Check the Breather Valve



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**Always follow the instructions in your repair manual when doing repair or maintenance work on Outdoor Power Equipment. Manuals can be found at the manufacturers website.**

A small engine breather valve is a one way check valve. It works by allowing high pressure to escape the engine crankcase, but close when crankcase pressure is lower than the outside ambient air pressure. This has the overall effect of a crankcase pressure that is lower than the outside air pressure. The volume in the engine crankcase is not constant because the bottom of the piston is part of that area and the piston is moving up and down as the engine is running. When the piston moves from bottom dead center to top dead center the crankcase volume increases and the pressure decreases. When the piston moves from top dead center to bottom dead center the crankcase volume decreases and the presure increases, and during this stroke the crankcase breather opens up and allows the high pressure to escape.

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36 thoughts on “How a Small Engine Crankcase Breather Works and How to Quick Check the Breather Valve

  1. Ive got a twelve hp motor the gas blows out the carb. But wont suck fuel to plug. If you spray fluid in plug hole the engine will fire and run. After you cut it off it doesnt suck the fuel back to the plug to start back.what can it be?

  2. In some breather valves you can see a wire mesh that stop any oil from leaving the engine. The oil collects on the wire mesh, and drips back down to the crankcase.

  3. (1) is it like a PCV valve where unburnt gases are channeled back into the carburetor to be burnt again, or is it just to relieve pressure?…(2) when you tip a lawnmower on its side, does oil come out of the vent line into the carburetor? if so, I assume this is not a good thing…

  4. i have a honda vtwin 24.5 hp and the air tube from crank into carb is blowing oil and making the engine smoke then the smoke stops for a bit then starts again…. i took hose away frim carb and let dangle but oil still came shooting out… any suggestions? Thanks

  5. One my old engines, why is this hose attached to the carburater? When I detach it, the engine runs better and I can't even start my lawn mower with it attached.

    note I don't care about the environment so if its an environmental thing I'm perfectly fine.

  6. I did the hose test as you instructed, and no air comes inwards (when i blow air in) through the diaphragm, but there's a tiny hole at the bottom of the inside of the breather where air does come in. Is that normal?

  7. The PCV is intended for releasing blow-by gasses from the crank case. It has little to do with crankcase volume as the piston moves. Piston rings do not seal perfectly. A small amount of combustion gasses leak or "blow by" the piston rings. This can cause pressure to build up. If it were for case volume it wouldn't serve much purpose being a one way check valve. The chamber pressure would simply oscillate between neutral and negative pressure. Blow-by pressure can become much larger than any slight pressure changes from changes in case volume and could be dangerous if high enough. It's probable unburnt fuel is present in this gas and could be a problem under pressure and for emissions purposes this gas is collected or sent back into the intake.

    Even small engines these days have emissions in mind and most route these gasses into the intake similar to vehicle engines.

    Good video.

  8. I have oil leaking out of the crankcase gasket on an 18hp ohv vtwin kawasaki vertical lawnmower engine. It seeps out pretty good when I turn the engine off. It makes good power, has the correct oil level, so I am kind of going in the direction of pressure build up. What do you think is causing this?

  9. when they stick,it will cause oil to be sucked into the exaust. I found that out. What a mess. Someone put sugar in the gas,not sure if it causes the valve to stick but I know what happends when that diaphram doesnt do its job any more.

  10. On my Vanguard 16hp v-twin, there are excessive gasses exiting the "crankcase vent" or breather as you call it. In a cars crankcase ventilation system, there is a breather (that sucks in fresh air) and a PCV exit to evacuate the fumes and pressure. I am trying to understand where my engine draws fresh air into the crankcase to create an airflow rather than an internal vacuum that sucks the oil out. A vacuum leak causes excessive crankcase oil to vent on my Vanguard (infamous for this). If the inside becomes a one way vacuum I can see how the oil will exit this valve. Or perhaps this valve has failed and is meant to separate fumes from liquids?

  11. Concise, to the point, jargon-free, and well explained with a great use of the small, clean engine. Good video. I just disassembled my Honda PCX to get to the engine and check the valve clearances, but when removing the crank case breather pipe out of the way, I could not remember its function clearly. I now have no need to check any more sources.

    Thank you. I wish all explanatory videos of this kind were as well put together.

  12. awesome vid. my question is i am getting oil into the crab covers my hand. its is an 18 horsepower briggs engine overhead valve engine. about 6 yrs old and a brand new carb i put on just 1 week ago. what do you suggest to start with.

  13. hi there, I bought a predator 13,000 11,000 watt running generator , it runs for about 10 to 15 minutes, then blows a bunch of smoke out and dies out. it starts right back up, any ideas on what would cause this, thank you

  14. I just had a new sleeve with piston and rings put into my 2005 Suzuki king quad. I had the head cleaned and valves and head machined. I started the engine and I have allot of blow by in the crankcase as well as allot of blue smoke coming out the exhaust pipe. can you help. Ron.

  15. I have kohler courage 6.5 vertical shafted engine, fitted to my dr trimmer tr4 model, the crankcase pipe is always blowing white air and oil out, and when this pipe is fitted back to the air filter back plate the engine loses power so i've pulled it off there but still have the issue of it blowing and spitting out oil.
    I have since fitted genuine kohler breather kit but it's no better and it still keeps blowing out continuously. Is there something else that could be creating too much pressure inside the crankcase?
    The trimmer seems to run ok starts really easy first pull. Maybe a little low at full power but i suspect it to be related to the blowing issue.

  16. I have a 4cycle Gas trimmer.  Oil is flowing out of the breather tube.  Anyone have a suggestion of what is wrong?  It runs but leaks oil everywhere!

  17. engine will not idle vacuum leak petcock clogged filter on the air leak into muffler smoky exhaust and engine runs roughly fuel clogged dirt oil leak or gasoline leak into muffler spark plug become carbon fouled dirt air idling seen black carbon fouled on spark plug a small valve pipe of a steam boiler cylinder of steam engine hot breathe pipe and breather tube , breather valve sometime engine roughly smoky because timer limit wear bore knife cutting rings piston bore limit wear many mileage bore …………..

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