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 surprised me.

So what’s wrong with a mower that’s hard to start when hot? Hard hot starting mower issue is most likely caused by 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 let’s take a look at all the simple stuff first before we go deeper.

This post covers the complete diagnoses and repair process, if however, you need video help, check out “Mower won’t start video”. It walks you through the complete process, from diagnosing no-start issues (including hot start issues) to repair.  

Check The Oil Level

OK, I know this sounds like it’s not relevant 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 to 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 “Spark test video”.

Spark

A 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.

What Is A Coil?

A coil is a solid-state unit that 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 windings. 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 the ground and so the positive voltage is made the jump from the plug electrode to the 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 the ground. So when you release the bail lever, you’re offering the voltage a perfect path to the ground, and since it loves a shortcut, 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 nonserviceable item. Examine the plug cap for arcing and the coil wire for chafing. If damaged it will cause an intermittent no start. Higher temperatures create higher resistance to the flow of voltage, that’s why coils usually fail when the mowers are hot, and start 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.

Check out the “Small engine tools page” to see the spark plug tester I use. It’s simple to use and gets the job done.

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

Spark Check

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

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 misdiagnosis.

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

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”.

5 Wire – Plug wire damage can be caused by old age, mice, or chafing of 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 handlebars or at the engine. If the bail lever isn’t pulling all the way the mower won’t start.

7 Switch – The bail 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 is – they’re easy to fit!

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 – On the underside of the coil, there is a single push on the wire connector, this is the coil control wire, usually on the underside of the coil.

Coil – Ordering a coil online is easy but you will need your engine number or look for a part number. Although all coils look the same, they’re not. Check out the Amazon link below for common coil types.

Amazon Lawnmower CoilOpens in a new tab.

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 the business card.

Engine May Be Overheating

This can cause serious damage, common reasons for an overheating mower: engine running lean; using the 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 as 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 components like coils and plugs will fail when they get too hot.

Is It A Mechanical Fault?

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”.

A compression test will rule a mechanical fault in or out. You’ll need a compression test gauge, you’ll find all these tools on the “Small engine tools page”.

Is It Getting Fuel?

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 setup.

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 an 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.

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 an 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 setup.

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.

Choke

Some automatic choke systems are prone to over-fueling 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 smoky – will likely need the carburetor cleaned or replaced.

12 Choke Off – When the throttle is set to run – the 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 an 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 Choke 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 the choke is lever-operated, if not adjust the cable. If you have the auto choke system – the choke plate should be in the closed position as per the picture when the engine is cold.

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

Replace 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.

Check out the Amazon common carburetors link below.

Amazon Lawnmower CarburetorsOpens in a new tab.

Related Questions

How do you fix a mower that overheats? Remove the engine cover, and using a hand brush or compressed air removes 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.

Lawnmower won’t start oil on a 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