Why Your Lawn Mower Blows Blue Smoke.




A Lawnmower blowing smoke

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It’s a beautiful day and you head outside to mow the lawn. Probably thinking that you could be doing better things. This feeling is compounded when you start to see smoke rising from your mower. Although this is annoying it pays to do a few simple checks before running off to the local fix-it shop.

There are a few things that can make your mower smoke and although it may appear to be something major it is normally a simple fix.

A lawnmower blows blue, white or black smoke for the following reasons.

  • oil reservoir has been overfilled.
  • The mower has been tipped on its side.
  • Choke needs adjustment.
  • The air filter is blocked.
  • The carburettor needs adjusting.
  • The spark plug gap is wrong.
  • The engine is worn.

The colour of the smoke is a clue about the problem and in my 30 years of commercial lawn mowing I have encountered all of them.

This is a simple list I would give to my employees to go through before they contacted me. If you run through this simple checklist you should be able to self-diagnose the problem, fix it, and get back to that lawn.

The mower blows out Blue or White smoke.

This is a sign that the engine is burning excess oil. The excess oil in the engine causes incomplete combustion which will create smoke and a carbon buildup. The engine may run like this for a while but eventually, it will foul up and become increasingly hard to use and eventually die.

Has the oil reservoir been overfilled?

The first thing you should do is check your dipstick. If you cannot find a dipstick then you have a two-stroke engine go to the bottom of this post and I talk about two strokes there.

If the oil is above the maximum full lever on your dipstick then is a problem. You will need to drain some oil out of the mower. There are a few ways to do this.

  1. Unbolt the sump plug – This is usually the engine manufactures recommendation. A word of warning here. Not all engines have a sump plug so if you cannot find it go to step two. Remember to access under the mower by lifting the front wheels up, not by flipping the mower on its side. (you don’t want to compound the problem). In most cases, you will need to remove the blades.
  2. Remove the dipstick and tip the mower on that side – I know that in step one I said don’t tip the mower, but it is ok if the dipstick is removed. As long as you do not tip the mower on the filter side you will be fine. It is a good idea to run the mower for a few minutes first (this makes the oil flow better). Drain out a bit of oil and check the level again. Do this until it is on the right level on the dipstick and then run the mower for about five minutes until the smoke clears.
  3. Use an oil extractor pump – This is the tool that most mower shops will use. I would suggest method two but if you are going to be doing this a lot then an oil extractor may come in handy in the future. This is a good one made by Briggs & Stratton on Amazon. Again run the engine for a few minutes before draining the oil.

If you are not sure how to use an extractor pump then the Briggs and Stratton website has a video that walks you through this procedure.

Do remember that if your mower was smoking you have probably fouled up the spark plug as well. You will need to clean or replace it.

Finding the sump plug.

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This is where you find the sump plug. I usually use a socket wrench without a socket to remove the plug.

Has the mower been tipped on its side?

If your mower has been tipped on its side then this could be the problem.

This can happen if you flip the mower without thinking if something tangles up on the blade or cleans under the mower. It is even possible for this to happen when you are mowing on a slope if the angle is great enough. This is more likely when you are mowing across a steep lawn rather than up and down.

If you flipped the mower with the muffler side towards the ground then you may have got oil in the muffler. Provided it wasn’t left like that for too long this should burn off. Let to mower idle for five minutes and see if it clears up.

If you tipped it on the air filter side then that is not as easy. First, check your air filter. If it is full of oil you are going to have to clean out the holder and replace the air filter. If you do not have access to an air filter immediately don’t worry too much, the mower will run ok without it while you finish the job. Just make sure to replace it before you use the mower again.

Now top up your oil, clean or replace the plug and you should be good to go.

Is the choke working correctly?

Check the cable attached to the on/off switch. Make sure it is not sticking in the choke position. This can cause too much fuel to get through which will create smoke and a badly running engine. Spray the top and bottom cable with Lubricating Oil and work the on-off lever.

Check to see if this has solved the problem.

If this does not solve the issue or you have an automatic choke then you will need to get this checked at your local mower shop.

Ware and tear on the engine.

If you can find none of the causes above it may be the engine and you will need a compression test.

This could be any excessive wear on the engine but it is usually a worn cylinder, rings, or the sign of a blown head gasket. All of these things will need to be done by an experienced mechanic so do not attempt to fix this unless you know what you are doing.

The mower blows out black smoke.

This is caused by the engine burning more fuel than air. Too much air leads to partial combustion and the excess fuel can turn into smoke.

The air filter may be blocked.

Pull off the air filter. Clean it by tapping it on the ground or by using an air compressor. Clean out the housing and replace the filter if it is too far gone. To test simply pull the filter out and start the mower. If the mower starts and doesn’t smoke then it is the filter.

If you have a foam air filter then you could wash it in warm water with a few drops of dishwashing liquid. Wring the water out and leave it to dry naturally. When dry I put it in a plastic bag with a little bit of Foam air filter oil then I squeeze the bag to evenly distribute the oil. This last step is not necessary but I prefer doing it as it catches dust better, especially if you work in dusty environments.

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This air filter needs replacing.

Your air mix on your carburettor needs adjusting.

This is normally a screw that you can adjust on your carburettor called the idle adjustment. You can turn the screw and it will adjust the idle on your engine. You can slow the engine down if it is idling too fast or vice visa.

How to Adjust your carburettor video

Here is a video on adjusting the idle screw

You may have an incorrect gap in your spark plug.

Pull out your spark plug. Is it carboned up?

If so clean the plug and double-check the gap in the plug.

s it the correct gap (or plug) according to the manufactures specifications?

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A badly carboned spark plug

I normally just throw a new plug into the mower if I have any problems. This is the first thing a mower shop does when you bring a lawnmower in. They are only a few dollars each so its always worth having one on hand.

A Faulty crankcase breather.

If you have ruled out everything else excessive oil consumption and smoke can be the sign of a blocked crankcase breather.

Unless you are handy with engines then you would probably want your local mower shop to do this.

How to replace an engine breather video

Two-stroke engines blowing smoke.

The first thing you would want to check with a two-stroke engine is that you have used the correct two-stroke oil at the right mix. If the oil mix is too heavy then it will cause the engine to smoke.

You do not want to just add petrol to dilute the mix because if you dilute the mix too much you will risk seizing the engine which is even worse.

You can check that you have the correct oil easily enough but there is no easy way to check the mix in your tank. I always say “if in doubt then throw it out” better safe than sorry.

There is normally a small fuel hose that attaches underneath the tank held on by a butterfly clip. Unclip that and drain your tank. Reattach it and put it in the correct oil/gas mix. If the engine stops smoking then you have found the problem.

If there is still smoking go through the list above starting at “Is the choke working correctly?”

Nobody likes a smokie mower and hopefully, by now you no longer have one.

Now that your mowers are fixed, you can get back to that lawn. Time to mow the lawn while daydreaming about all the fun things you could have done today instead of fixing a mower.

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