Mower Hard to Start When Hot

Stop to empty the grass bag and the mower won't restart? You're not on your own, I had this exact problem with a briggs engine and the cause of the problem surpised me.

So what's wrong with a mower that's hard to start when hot? The fault is most likely a failing coil, but there are other possibilities:

  • Plug faulty
  • Engine flooded
  • Low oil level
  • Engine overheating
  • Plug wire faulty
  • Auto choke faulty
  • Carburetor faulty
  • Compression 
  • Valve fault

Now lets take a look at all the simple stuff first, before we go deeper.

 

 

 

Check The Oil Level

Ok, I know this sounds like it's not revelant but, some mowers won't allow the engine to start/restart if the oil level is low - it's designed that way, helps protect the engine from a critically low oil level.

 

The oil should be checked regularly, every time you fill the gas. Most mowers will take from empty .6 lt of 5w30 or 10w30. Mowers don't like to be overfull either, so add just a little at a time. If in doubt, Check out "Lawn mower oil level check".

 

 

Check The Spark Plug

Plugs come in a variety of lengths and heat ranges. If the wrong plug is fitted to your mower it can cause issues including hot start failures. It's a good idea to check the correct plug code with your engine maker.

 

It's always worth having a spare plug, it allows you troubleshoot quickly, by replacing the plug with a known good one. Sometimes simply changing the plug will fix the problem, and you only invested a couple of minutes. Check out "Coil & spark plug test".

 

Spark Plug


Mower Spark Plug

Spark plug is the most likely cause of a hot starting problem and is the easiest problem to solve. 

A new spark plug should be fitted at the start of every season.

Spark

Spark plug is the most likely cause of a hot starting problem and is the easiest problem to solve. 

A new spark plug should be fitted at the start of every season.


Mower plugs

What Is A Coil?

The coil is a solid state unit which is dedicated to producing a voltage at every revolution of the engine. It works hard and is exposed to high temperatures, it lives right above the cylinder head next to the flywheel.

 

Coils are made from copper wire, wound around a metal core, known as a winding. Most coils will have two independent winding's - primary and secondary.

 

How a Coil Works

The flywheel on the top of the crankshaft has a magnet attached and every time it passes the coil, it creates a voltage in the winding's. A transistor built into the coil controls the spark by opening and closing the circuit. 

 

The high voltage travels along the plug wire searching for ground. The spark plug provides an almost perfect path to ground and so the positive voltage is made jump from the plug electrode to ground, this is where the spark occurs.

 

The process takes milliseconds to unfold and is repeated every revolution for as long as the engine is running.

 

 

Coil Control

A mower engine is shut down by stopping the voltage reaching the plug, this is done by offering the coil voltage a shorter easier path to ground. So when you release the bail lever, your offering the voltage a perfect path to ground, and since it loves a short cut, the engine shuts off.

 

If the coil control wire is chafing off a ground source, the engine will not run or run intermittently. 

 

The coil is a non serviceable item. Examine the plug cap for arcing and the coil wire for chafing. If damaged it will cause intermittent no start. Higher temperatures create higher resistance to the flow of voltage, that's why coils usually fail when the mowers hot, and starts working again when the engine cools.

 

The enemy of electrical systems is moisture, so a mower should always be kept indoors in a ventilated area.

 

 

 

The Coil & Plug Test

In the following guide we will, test the spark plug, coil wire, plug wire cap, bail lever and the stop/start switch. You won't be surprised to know that there is a special plug spark tool, called an in line spark testing tool.

 

The tool loads up the coil, wire, cap and plug and so is a prefered way to test for spark. Anyway, we'll do it MacGyver style.

 


Spark Check

Tools


mower tools

Now turn the key, if your mower doesn't make any noise at all-follow this link. If it's cranking over but won't start-Follow this link.

1 Remove


mower spark plug

Now turn the key, if your mower doesn't make any noise at all-follow this link. If it's cranking over but won't start-Follow this link.

2 Replace


Mower plugs

Replace the plug with a test plug and check for spark again.

3 Pull


mower starting

The helper now attempts to start the mower while you watch the plug spark. 

4 Cap


mower coil

If you have no spark or it's poor, you may have a faulty: Spark plug wire; Plug cap: Bail lever cable: Short circuit of coil control wire: Coil damage.

If your spark is good and the problem still persists, then check out "carburetor cleaning".

5 Wire


mower spark wire

Plug wire damage can be caused by old age, mice or chafing off the mower body. 

If all looks ok move on and check the bail lever and switch.


Bail Lever

6 Bail


mower bail lever

Check that the bail lever cable is tight. If not adjust it.

Most models will have an adjuster at  handle bars or at the engine. If the bail lever isn't pulling all the way the mower won't start. 

7 Switch


mower bail cable

The bail lever lever is connected to the engine brake which usually incorporates a simple on off switch.

When the lever is released a brake block pushes against the flywheel, slowing the engine down, at the same time, the simple ground on off coil control switch is operated. Check that the cable is operating the assembly.


Replace Coil

8 Coil


mower coil

If all checked out okay, then remove the pull start assembly, and replace the coil.

It's not uncommon for them to fail, I replace lots and the good news- They're easy to fit!

9 Remove


mower coil

Remove these two bolts and remove push on wire connector. Coils are specific to each model, so check your engine type code before ordering.  

10 Wire


Mower coil wire

Usually, on the under side of the coil, there is a single push on wire connector, this is the coil control wire, usually on the underside of the coil.


Mower coil

Ordering a coil on-line is easy but you will need your engine number. Although all coils look the same, they're not.

11 Adjust


mower coil adjusting

Place a business card between the flywheel and coil. This creates just the right air gap.

Push coil snug against the business card and tighten the two bolts. Fit push on wire connector. Reassemble & don't forget to remove business card.

Spark Check


Tools

For this test you will need a new plug, plug spanner, insulated pliers and a kind helper.

 


Mower tools
mower plug test

1 Remove

Hold the plug against the metal of the engine (Ground) using the insulated pliers, be sure to ground it well as poor grounding will lead to misdiagnoses.

2 Replace

Replace the plug with a test plug and check for spark again.


mower plug
mower pull start

3 Pull

The helper now attempts to start the mower while you watch the plug spark. 

Coil


4 Cap

If you have no spark or it's poor, you may have a faulty: Spark plug wire; Plug cap: Bail lever cable: Short circuit of coil control wire: Coil damage. If your spark is good and the problem still persists, then check out "carburetor cleaning".


Mower plug cap
Mower coil

5 Wire

Plug wire damage can be caused by old age, mice or chafing off the mower body. 

If all looks ok move on and check the bail lever and switch.

Bail Lever


6 Bail

Check that the bail lever cable is tight. If not adjust it. Most models will have an adjuster at  handle bars or at the engine. If the bail lever isn't pulling all the way the mower won't start. 


Mower Bail lever
Mower Bail lever Cable

7 Switch

The bail lever lever is connected to the engine brake which usually incorporates a simple on off switch. When the lever is released a brake block pushes against the flywheel, slowing the engine down, at the same time, the simple ground on off coil control switch is operated. Check that the cable is operating the assembly.

Replace Coil


8 Coil

If all checked out okay, then remove the pull start assembly, and replace the coil. It's not uncommon for them to fail, I replace lots and the good news- They're easy to fit!


Mower coil
Mower coil

9 Remove

Remove these two bolts and remove push on wire connector. Coils are specific to each model, so check your engine type code before ordering. 

10 Wire

Usually, on the under side of the coil, there is a single push on wire connector, this is the coil control wire, usually on the underside of the coil.


Mower coil
Mower coil

Coil

Ordering a coil on-line is easy but you will need your engine number.

Although all coils look the same, they're not.

11 Wire

Place a business card between the flywheel and coil. This creates just the right air gap. Push coil snug against the business card and tighten the two bolts. Fit push on wire connector. Reassemble & don't forget to remove business card.


Mower coil

Engine May Be Overheating

This can cause serious damage, common reasons for an overheating mower: engine running lean; using wrong plug or fuel type; air cooling fins obstructed. 

 

Running Lean

A lean running engine is lacking fuel or getting too much air - the air/fuel ratio is off. The extra air could be from a carburetor fault or a vacuum leak somewhere in the engine.

 

 

Ethanol Gas

Small engine manufacturers recommend regular gas or e10 ethanol. E15 and e85 burn hotter and will damage the engine, worst of all, it voids your warranty.

 

 

Air Cooled

Small lawn mower engines are engineered with clever cooling fins that help cool the engine by having a larger surface area exposed to the atmosphere. These fins get packed with old dry grass, which if not cleaned, start to act like insulation, causing the mower to overheat.

 

The solution is simple enough, remove the plastic engine covers and clear the grass with compressed air.

 

 

Low Oil Level

Oil is used to cool as well as lubricate. When the oil level is low, it will cause engine temps to rise. Engine Componants like coils and plugs will fail when they get too hot.

 

 

Is It A Mechanical Failure?

Total failure is rare but it does happen, problems often only show up then. Coincidence? No, metal expands as it heats, problems such intake manifold cracks/gaskets, cylinder head gaskets tend to leak when the engine is up to temperature, bearings and valve train uglies are at their worst then too. 

 

Cylinder head and intake gaskets are not expensive or challenging to replace, but anything deeper in the engine, may not be economically viable to repair. Often a whole engine is cheaper, faster and as Spock would say "the logical choice". 

 

 

 

 

Is It Getting Fuel

Difficult hot starting can be caused by a badly adjusted throttle cable, which can cause flooding. In some cases operator error may be to blame, generally choke is not needed on a hot engine.

 

Your mower may be fitted with auto choke, manual choke or a priming bulb. As you know the function of a choke or priming bulb is to enrich the fuel mixture, so a cold engine starts smoothly. 

 

A failing carburetor float needle will supply too much fuel, so when you shut it off to empty the grass bag, the gas floods the engine and it won't start. Testing your float needle is easy, check out "carburetor troubleshooting" (internal link)

 

The choke does this by restricting the amount of air entering the carburetor. The priming bulb does this by injecting extra fuel. 

 

AFR

Gas engines run best when the ratio of air to fuel is 14.7 to 1. Meaning 14.7 parts air to 1 part fuel, also known as air fuel ratio (AFR). Using the choke or priming bulb enriches the fuel mixture which counteracts the lean condition caused by the dense cold air, on a cold start.

 

 

Auto Choke

Most mowers will typically have a lever to control choke, more recent engines from Briggs & Stratton and Honda offer a thermostatically controlled automatic choke system. It's a simple set up.

 

A thermostat, positioned beside the muffler pushes open the choke plate progressively as the engine heats. The system is a great idea no doubt, no more fiddling around with choke levers or priming bulbs. 

 

Some of the B&S auto choke carburetors tend to supply too much fuel to the cylinder when hot starting, this floods the spark plug causing a no start. They have since modified the carburetor. So if have a auto choke briggs, with the hot start problem, go ahead and swap out the carburetor. 

 

In the guide below we will check that the choke is working correctly. The test will apply to both auto and manual choke systems, but not for the priming bulb type carburetor.

 

 

Carburetor Choke


Choke

Some automatic choke systems are prone to over fuelling when hot starting.

If your mower has a priming bulb this test does not apply. An engine fitted with a priming bulb type choke that's hard to start hot, and is smokey - will likely need the carburetor to be cleaned or replaced.


Mower auto choke
mower choke mower choke

12 Off

To test choke plate type,  remove the air filter. Here I have removed the complete air filter housing for demo purposes.

When throttle is set to run - choke plate should be open. Often a choke plate may be partly closed even though the throttle lever is set to run, adjusted cable.

Your mower may not have a choke lever, you may have auto choke, this test will still apply. Check the choke plate when hot, it should be open. If not check for binding in the linkages / failed thermostat.

13 On

The choke plate in the closed (on position). This is the correct position for starting a cold engine.

Check that the choke closes fully when choke lever operated, if not adjust cable. If you have the auto choke system - the choke plate should be in the closed position as per picture when engine is cold.

If not check for binding or fault with the thermostat choke control unit - fitted against the muffler.


mower choke on mower choke on
Image

Replace

It's also quite common for carburetors to fail, causing either too much or too little fuel. Cleaning the carburetor is always a good plan, and if this doesn't help, replace it.

Replacement Briggs & Stratton carburetors are inexpensive and easy to fit.


Carb Choke

Auto


mower carburetor

Some automatic choke systems prone to over fuelling when hot starting.

If your mower has a priming bulb this test does not apply.

An engine fitted with a priming bulb type choke that's hard to start hot, and is smokey - will likely need the carburetor to be cleaned or replaced.

12 Choke Off


mower choke lever mower choke

Remove these two bolts and remove push on wire connector.

Coils are specific to each model, so check your engine type code before ordering.  

13 Choke Off


Mower choke lever Mower carb choke

The choke plate in the closed (on position). This is the correct position for starting a cold engine. Check that the choke closes fully when choke lever operated, if not adjust cable.

If you have the auto choke system - the choke plate should be in the closed position as per picture when engine is cold.

If not check for binding or fault with the thermostat choke control unit - fitted against the muffler.

Replace


mower carburetor

It's also quite common for carburetors to fail, causing either too much or too little fuel.

Cleaning the carburetor is always a good plan, and if this doesn't help, replace it.

Replacement Briggs & Stratton carburetors are inexpensive and easy to fit.

Related Questions

How do you fix a mower that overheats? Remove the engine cover, and using a hand brush or compressed air remove the dry grass clippings from the engine cooling fins. Check also that you're using the correct gas, regular gas is best for small engines, e10 is ok but e15 or e85 will cause the engine to overheat.

 

Lawn mower won't start oil on spark plug? The most common reason for oil on a mower spark plug is too much oil in the engine, but there are other possible causes:

  • Mower turned on its side
  • Head gasket failure
  • 2 Cycle gas used
  • Engine worn out
Auto Technician and Writer at | Website

John Cunningham is an Automotive Technician and writer on Lawnmowerfixed.com. I've been a mechanic for over twenty years, I use my knowledge and experience to write "How to" articles that help fellow gear-heads with all aspects of mechanical repairs, from lawn mowers to classic cars.