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White Smoke From My Engine

Updated on October 7, 2012

Easy Lawnmower Repair

"Clouds of white smoke."

This is one of the common complaints we see at the small engine shop in the Spring. The complaint is usually the same, "I put the Tractor/mower/tiller/snowthrower up for the season and it was fine all of last year. This year I take it out and I start it up and after a couple of minutes it starts blowing white smoke very heavily. Are my rings shot?"

Generally billowing clouds of white smoke are from one of two things. Either the crankcase breather has failed or the oil is contaminated.

The crankcase breather vents the gases and excess pressure from the crankcase and sends them through the carburetor to be burnt in the combustion process. Some breathers are nothing more than a reed valve that opens and closes as the pressure changes inside the crankcase from the piston's movement. If this reed breaks, bends or otherwise fails, then excess oil is pulled into the carburetor and the result is a lot of smoke. Other breathers are screen or filter type and when they become too fouled or broken down to function, same result, lots of smoke.

Diagnosis is done by looking for two things, oil in the air cleaner/carburetor and black oil soaked or heavily carboned spark plugs. Since oil is being fed directly into the cylinder, the plug will foul very quickly and probably also affect engine performance. Breathers can fail at any time, often they will fail over the winter as the oil that's built up on and around them solidifies and just generally plugs things up.

Repair is a pretty simple matter on many engines, just replace the breather. You can try turning the reed around on the reed type breathers, but personally I just replace them. Sometimes all they need is a cleaning though, so give that a try. Most breathers will run from just a couple dollars for reeds to maybe twenty dollars although a few that are incorporated into the valve cover may be upwards of thirty dollars.

Reeds and complete breathers are available from most small engine shops for most engines. Online suppliers such as Jack's Small Engines, and M&D are excellent and reliable sources for parts as well.

To replace a breather, follow the tube or hose from the back of the carburetor or air filter assembly to where the breather is mounted on the crankcase. Most are simply attached with two screws but may be under the flywheel requiring flywheel removal. If it's incorporated into the valve cover, just replace the valve cover. Some are cartridges that plug into the valve cover, very easy to replace.

The other common cause for heavy white smoke, especially in mid-size Briggs and Stratton and Kohler engines, (12-20hp) is from contaminated oil. The most common contaminant will be gasoline that has leaked from the carburetor. We have had at least one engine that had a crankcase full of water, this was probably a case of sabotage from an irate neighbor.

The diagnosis is simply to examine the oil. It should have little to no smell of gasoline and should not be overly thin or muddy brown, gray, white or chunky like spoiled milk. Do not under any circumstance attempt to start an engine with oil that fits any of these criteria. The most common cause of mid-size Briggs and Kohler engine catastrophic failure we see in the shop is from gasoline diluted oil causing the rod to overheat and break just above the crank journal.

Gas will get into the crankcase when you have a carburetor that is leaking past the needle. This leak is generally caused by either a float problem or other problem keeping the needle and seat from sealing. If there is gas in the crankcase, then the recourse is a carburetor rebuild or replacement followed by an oil change. The reason that this is such a common Spring problem is that if you leave any gas in the carburetor, it will evaporate and leave behind a varnish coating that can prevent the float assembly from functioning. Also, and even more common, the ethanol in today's fuels will ruin needles and seats, preventing them from sealing the flow of fuel off, which causes the carburetor to overflow and leak into the crankcase. Gas can also get into the crankcase from a fuel pump that's leaking, so if you have a pump, that needs to be checked as well.

These items are often the result of improper storage. I can't stress enough the importance of proper storage of outdoor power equipment.

If there is water in the oil, then several crankcase flushes with kerosene and a few oil changes may clean the engine out. However engine disassembly may also be called for to remove all the muddy deposits and ensure that the oil channels are cleaned out.

These two items are the most common causes of a lot of white smoke. We do see a few OHV engines come in and have blown head gaskets. Usually this is on a Kohler and often the oil is leaking down onto the exhaust manifold and not into the cylinder. A leaking valve cover gasket will also do this.

If the plug isn't fouled out, and the oil is normal, then check closely for a leak. Spray carburetor cleaner on the head to clean all deposits off and then run the engine for a bit. Then check for a leak again. Some talcum powder thrown onto the surface will help to spot a leak as well.

Hopefully one of these will be the solution and you won't be purchasing a new engine. If you need further help, think about posting your question on in the Small Engine category. Good Luck!

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.


Submit a Comment
  • profile image

    James Duguid 

    8 years ago

    Was ready to pull engine and replace piston rings.. Was just a leaky valve cover gasket.. Thanks

  • PK Jones profile imageAUTHOR

    PK Jones 

    8 years ago

    Hi, most likely your carburetor is going to need to come apart and be rebuilt. Either the float is sticking or leaking or the needle and seat are leaking. Either way it's going to be an issue in the carburetor and it will need to be repaired before you try and run the engine.

  • profile image


    8 years ago

    why is gas leaking into crankcase on Honda generator?

  • PK Jones profile imageAUTHOR

    PK Jones 

    8 years ago

    Hi, I really can't do diagnosis in this format, you'd be better to post a question on the site in the Small Engine category. But I will offer this advice.

    Don't try to use a heavier oil, it sounds more like you have a crankcase full of gas. Don't try to run the engine like this, it must be fixed first. If your air breather has oil in it, then probably it's a bad breather, if not, then you probably have gas in the oil. If you have a fuel pump, it can also cause smoking but won't dump a lot of oil out the exhaust. Based on your symptoms, I think you have a carburetor issue.

  • profile image


    8 years ago

    Hey, thanks.

    bought a recon from Home depot for $100 5hp b&s.

    I will go through all of these and see what it is.

    My main issue is the oil dripping from the exhaust (sorry, simi-car guy, not much on mowers). My first mower. first house.

    There is large amounts of smoke.

    last time I mowed was like a month ago,no issue, no funny noises, filled oil right after mowing.

    I move it today out of the garage, there is a small spot on the ground. I begin to assume I over filled. I crank up the mower, tons of smoke. So much that someone driving by stopped and starred. I barely could see them. I look at the exhaust, black liquid bleeding out, I could see that it was black stuff being washed out with brand new oil.

    The stink wasn't to bad. If that makes sense. Didn't smell gassy or exhaustish. Smelled like oil. like new oil burning.

    mower bogged down a couple times. While not really cutting thick grass. thin, maybe 1 inch above normal cut level for me (mid height setting on mower).

    I have been around people who had a similar (looking to me) issue, there was a bell ring sound with their mower. Not the case with mine, just sounded wimpy but quite normal.

    I am thinking I over filled the oil last month.

    Again I know very little about mowers, or pretty much anything that deals with a diagnosis. So I am trying to think of all I seen, heard and smelled.

    i will pull plug, drain oil and make sure it filled to the line or slightly under.

    similar to your carb cleaner idle check, I have seen an old man do it with wd40 and it would nearly kill the mower when he hit the spot of the leak. I assume you are not suggesting that because it takes longer for wd40 dry and could potentially smoke up from the heat.

    I will try a thicker grade of oil im using 4sae 40 (hope that isn't the issue). I may get some of that sap looking stuff and mix it with a thicker oil. if that stops the smoke or reduces it, then for certain I have a leak internally.

    or does this sound completely insane for me to try?

    Will come back and see if you respond.


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