How to service a 4 stroke lawn mower

Recently one of my neighbors was having problems starting their 4 stroke lawn mower. I went over to see if I could help and it was one very neglected grass cutter. I was able to get it going for them after 1/2 hour or diagnosing the problem which turned out to be low engine oil.

So I thought I would write up a simple post on how to service a 4 stroke lawn mower once a year so you will have easy starting and a long-lasting lawn mower.

Tools and parts needed

  • 19mm spark plug remover socket.
  • Socket set.
  • Screwdrivers of various sizes.
  • Clean rags.
  • Engine oil vacuum pump.
  • New air filter or make your own small engine air filter.
  • New spark plug.
  • 10-40 grade engine oil 4-stroke.
  • Degreaser – May not be required but handy.
  • Wire brush.
  • STA-BIL Start your engine spray.

How to service a lawn mower / Briggs and Stratton, Rover, Victa

Lawn mowers are a very easy engine to service. They only have 5 major parts to them.

  • Engine
  • Fuel tank
  • Carburetor – Throttle
  • Air filter
  • Blades

If any of the above is faulty you will have issues starting your lawn mower. We will break down each component to show you how to service your 4 stroke lawn mower yourself.

Lawnmower engine service

How to service a 4 stroke lawn mower

There are 2 main parts in servicing your 4 stroke lawn mower engine. They are engine oil and spark plug. I like to replace these every year, but you will be fine replacing them every two years if you have a small yard.

Start by removing the spark plug and replacing it with a new one. These are smaller than car spark plugs and are often 19mm in size. Pull off the spark plug cap and unscrew the spark plug.

Have a look and the condition of the spark plug. If it is black and sooty you could have a problem with your air filter letting in dust or bad piston rings due to wear. It should be a nice brown color on the white insulator section. If it is black and you don’t have a spare you can clean up the metal spark gap contacts with some 300 grit sandpaper. This will help in starting your lawnmower.

Engine oil should be replaced every year. Over time the engine oil will be contaminated with carbon from the piston fuel-burning process. This carbon riched oil will break down seals and cause engine wear on the piston bore and rings.

Lawnmower engine service 4 stroke.

An easy way to get the oil out of the engine is you use an oil vacuum pump. The hose is put down the oil fill tube to the lawn mover. Once you have pulled out all of the oil you can refill it to the appropriate level on the dipstick. Don’t overfill the engine oil as you will have trouble starting your lawn mower.

Fuel tank service

IF you haven’t used your lawnmower over winter and it has 6-month-old fuel you may have trouble starting it. If so drain the fuel and replace it with new premium fuel. The fuel stabilizer will help for long-term storage.

You can use a manual vacuum pump to suck out the fuel or simply tip the lawn mower upside down to drain the old fuel into a container.

If there is a fuel filter attached check to see if it has captured any particles. Often dirt and dust will be filtered out.

Carburetor service.

There is not a lot to do with the carburetor and throttle cable. Check that there is the correct amount of slack in the throttle cable. Lubricate this cable with WD40 or another lubricating spray.

Check the fuel primer push button that it is not cracked or deformed. These will break down after around 10 years if left out in the weather.

If you don’t have a fuel filter attached sometimes dirt can find its way into the carburetor and block up the jets. This will require a strip-down of the lawn mower carburetor and a full clean-out. It may be best to take it to a lawnmower service center if the carburetor is clogged up and you are not confident to do it yourself but it is simple.

A symptom of a blocked carburetor on a lawn mower is starving for fuel. If you have low power and blade speed you will often have a fuel problem or a blocked air filter.

Check the fuel by pressing the fuel primmer button while the lawnmower is running it you increase in speed and power. This is a quick test to diagnose a blocked or restricted carburetor.

Removing the air filter temporarily will prove if the air filter is blocked. Below is a great video on how to clean the carburetor on a lawn mower.

Air filter service – small engine foam air filter material

Lawnmower won't start after sitting for a while
The air filter is usually located on the top or side of the lawnmower.

The air filter on your lawn mower should be rinsed out or replaced every 2 years. To clean an air filter rinse it in a bucket of warm soapy water and fully dry in the sun but only the sponge type of filters. Spray it down with air filter oil. You can also use regular motor oil on your air filter but don’t use too much or it will impede airflow. Squeeze out the excess, you only need a fine coating.

Some lawn mowers use a cartridge air filter. These will need to be thrown out and replaced every 2 years or earlier if used in dusty areas.

Making a foam air filter

Making a foam air filter for your lawn mower DIY

Most lawnmowers use a universal air filter. These are easy to make from a thick piece of foam sponge. It is the same foam used for washing your car. Use the filter housing as a template and use a sharp knife to cut around the edges. They are great small engine foam air filter material.

Then trace out the center hole for the carburetor intake. I used a 30mm hole punch but you can cut these out with scissors or a sharp knife.

small engine foam air filter material

Make sure it all fits and then coat the DIY air filter with some air filter oil or use the same engine oil as your lawnmower. Congratulations you just saved yourself some money.

Checking cutting blades

Every year check over the cutting blades. Over time the lawn mower blades will become worn, and dull. They tend to lose the ability to throw the grass into the catcher when worn.

Make sure they have the fins that will create the turbulence required to move the grass into the catches. The blades over time will wear especially if you are mowing over rocks, sticks, roots, and tough grass.

To replace the lawnmower blades you will need a socket set and a wire brush to clean up all of the dirt that will form over the bolt.

Lawnmower won’t start after sitting for a while

If your lawn mower won’t start after sitting for a while then it is usually caused by dirty old fuel. Empty the fuel from the lawnmower tank and replace it with fresh fuel. Clean the spark plug while you are at it for a strong fresh spark.

Check for spark by removing the spark plug and connecting it to the lead. Place the spark plug on the metal engine frame. Pull the starter cord a few times and look for a spark. I don’t recommend holding onto the spark plug while pulling the cord as you will get a nice jolt.

Screw in the spark plug and tighten to the required torque. Make sure to prime the fuel by pressing the primer plunger a few times. It should start to firm up once it is primmed. Pull the cord a few times and then prime some more if it won’t start.

I use a product called Stabil start your engine. It comes in a spray can and is like a high octane boost for your fuel and engine. Just spray it into your carburetor with the air filter off. I guarantee if you have a good spark, your lawnmower or whipper snipper will start right up.

How to fix a lawn mower that won’t start

Most pull starting problems with lawnmowers come down to fuel, spark, and oxygen. If you are missing any one of them then your lawnmower won’t start.

  • Make sure to have fresh fuel, anything older than 6 months should be discarded safely.
  • Clean and inspect your air filter yearly.
  • Replace your spark plug every 1-2 years. Make sure it is clean and provides a good spark.
  • Use some STA-BIL Start Your Engine spray.

Electrical problems are rare on lawnmowers as they are very simple machines. Sometimes on 4-stroke lawnmowers, there is a low level engine oil switch that can shut down the electrical circuit and prevent it from starting. Top up the oil level or replace the sensor if it is faulty.

Both 4-stroke and 2-stroke lawn mowers require priming of the fuel. Most times you only need to prime the fuel bulb 3-4 times. I have repaired an older Briggs and Stratton lawnmower that needed priming 6-8 times before the grass cutter would start.

You can flood the carburetor and this is bad as well so go easy on the priming. Read the owner’s manual, but as the lawnmower gets older you may need to prime the fuel an extra pump or two.

Check or fuel or air leaks, this can casue a lean or rich fuel mixture which will make your lawnmower hard to start.

Sometimes there is no such thing as an easy start lawnmower. You have to pull and pull and pull. But if you get the fuel, spark, and air mixture correct you should have no problems starting any lawn mower on the 1st or 2nd pull.

Lawn mower died while mowing and won’t start

First, check for fuel in the tank most often you have run out of fuel. Top up if required. If you have good fuel then next check the spark plug lead that it hasn’t fallen off the plug. If it is on good then remove the spark plug and check if it is dirty. A black and dirty spark plug will make it hard to start. Clean the spark plug with some 300 grit or finer sandpaper.

If you have fuel and spark then check the air filter that it is not clogged or disintegrated. Over time these foam filters can break down and clog the carburetor. Replace the filter and clean the carburetor.

Check the oil level in a 4-stroke engine. Sometimes these engines have a low oil level switch. This will cut power to the engine. Top up the lawnmower engine oil to the recommended dipstick level. Most lawnmowers use 10-40 engine oil.

Check out this more detailed post on how to fix a lawnmower that runs for a while then dies.

Lawnmower starts then stops

If your lawn mower is constantly starting and then stops it is often a fuel problem. I have seen very dirty carburetors with this symptom. It is literally starving for fuel. Pull apart the carburetor and clean out any debris. Flush out the fuel tank, and clean the fuel filter if it has one.

Most often you will find mud and grass clippings inside the carburetor that needs removal. Flush it with carburetor cleaner and poke out any small holes with a paper clip. Screw the carburetor back onto the lawnmower. Replace any seals as needed.

Lawn mower pull cord stuck

How to fix a lawn mower that won't start

Sometimes the lawn mower pull cord can become stuck. There are three possible reasons for this.

  • Jammed blades
  • Rusted spring or ball bearings in the pull cord starter.
  • A broken pull starter cord

Check that the rotor blades are not jammed by turning off the fuel, setting the throttle to stop, remove the spark plug cap, and tilt the lawn mower on the side. Slowly rotate the cutting blades to see if they move. If they don’t you either have something jamming the blades or you have a seized engine.

Remove the offending object. If it is a seized engine go buy a new or second-hand lawn mower as it will be cheaper to replace rather than repair.

If you have an issue with the pull cord starter you will have to take off the top starter. Often the ball bearings in the pull starter can become rusted and will need some lubrication.

Broken pull starter cords and springs can also be a problem over time. I have replaced my cord with some basic thin rope found at my local hardware store. It is much cheaper than the real thing but does the same job.

Conclusion

I have seen well-maintained lawn movers last over 30 years. For a machine that is often neglected, stored out in the weather, and smashed around, lawn mowers are built tough.

Stick with well-known brands like Briggs and Stratton, Victa, Rover and you will have no problems getting parts. Some Chinese lawn mowers are cheap but they are often low on power and hard to start. Parts are also a problem.

Be sure to service your 4 stroke lawn mower every year and you will have perfect running and easy starting every time.

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