Chainsaw Won’t Start Hot – Fixed!

By: Author John Cunningham. Published: 2023/07/17 at 6:19 pm

What a pain in the jacksie… should have kept her lit! Not to worry; we’ll get this saw running full tilt in a jiffy.

A chainsaw that won’t restart after a short rest period typically suffers from a vapor lock. Allowing the saw to cool usually solves the problem. However, there are other possible reasons, and they include:

• Ignition system fault
• Spark plug gap
• Flooding
• Vacuum leak
• Compression issue

In this post, we’ll look at common reasons your chainsaw won’t start when hot, how to diagnose it, and how to fix it.

Index

• Vapor lock
• Ignition system issues
• Engine flooding
• Vacuum leak
• Compression issue

Vapor Lock

Why does it happen? Vapor lock is caused by excessive heat, and all small engines are susceptible, some more so than others. The engine is the obvious source of heat; by their nature, small engine gas tanks will never be far from a heat source. But friction plays a part, too, especially in chainsaws – bar and chain add a ton of heat if the chain is blunt or there’s an oiling issue.

Typically, vapor lock happens when a chainsaw with a low fuel level is allowed a short rest after working hard. The now hot but stationary saw doesn’t have the cooling effects of the rotating flywheel fins to help cool it; instead, that heat builds, and the heat soak boils the fuel and vaporizes it.

To confirm, allow the saw to sit in a cool breeze with the gas cap removed until cool. Now attempt to prime her again; if the primer now fills with gas, you can bet vapor lock was the issue.

Tips to prevent vapor lock:

• Idle saw – idle saw for a minute after heavy use before shutting down.
• Use low or no ethanol-content gas – most manufacturers are OK with E10.
• Use gas stabilizer – gas stabilizer helps keep gas fresh and prevents gumming.
• Keep saw clean – clean the saw after use, and keeps dust and dirt off the cooling system.
• Keep the chain sharp and bar oiled – a sharp and well-oiled chain means less work for the saw.
• Air filter clean – allow the saw to breathe fully by keeping that air filter clean and clear.
• Avoid running gas tank low – more liquid is less likely to boil, more is more.

Excessive heat promotes vapor lock; some common root causes of excessive engine heat and persistent vapor lock include:

• Prolonged heavy cutting without rest
• Dirty air filter
• Clogged airflow system
• Wrong gas type
• Saw running lean
• Wrong spark plug
• Bad coil
• Blunt chain
• Faulty bar oiler
• Faulty chain brake
• Chain too tight
• Faulty clutch

Ignition System Issues

An ignition system is responsible for making a strong spark and delivering it to the combustion chamber at just the right time and doing so without missing a beat.. ever! It’s an important job, then, and not surprisingly, if it falters even a little, the symptoms show up fast.

1 Spark Plugs

Spark plugs are simple but important links in a chain that won’t tolerate any failures. The plug consists of a center electrode, ground electrode, and outer ceramic insulation. The plug’s job is to encourage the coil’s current to jump its gap and thereby produce a spark.

In simple language, the plug’s job is to get electrocuted 200 times per second and suffer extreme temperatures inside the combustion chamber.

So what can go wrong with spark plugs? The wrong plug type is a common cause of hot start issues, but so too is a poor spark plug gap. As the metal expands, it causes an already small spark plug gap to close, causing a no-start when hot.

And that’s important; the ground electrode of the spark plug lives in a hostile environment with gas and oil and heat and fire and soot; a plug is designed to get hot enough to burn off these contaminates, fitting a cooler plug will mean it can burn off these contaminates and may foul when restarting.

Of course, a plug can foul for other reasons, and the simplest among them is incorrect gap size, too small, and the spark isn’t right enough and too big, and it may struggle to produce a spark at all.

How to diagnose spark plug issues? First, check your owner’s manual to ensure that the correct spark plug is fitted; the code on the ceramic insulator will specify the plug code.

Next, remove, inspect the plug, and gap the spark plug; you may find the links below helpful.

2 Coil Issues

The coil is where the magic happens; it transforms a low voltage into a high voltage capable of igniting our fuel mixture.

Three issues are common with coils; they are:

How to test the ignition system? Small engine ignition systems are basic, and testing is easy; typically, they either work or they don’t. But occasionally, we’ll meet the coil that likes to fail only when hot; it’s not uncommon, just not the more typical coil failure symptom.

1 Coil wire issues – I like to begin by visually checking the plug wire and spark plug cap boot.

2 Coil failure – Next, we’ll run a spark check. We’ll test it using two methods: toolless and then with an inline spark tester.

The caveman-style toolless method works fine for spark plugs but isn’t conclusive, especially when diagnosing an intermittent coil, so I prefer the inline tester.

Anyhow, warm up your saw and get it to fail if not already exhibiting the no-start condition, and here’s how we check spark with and without the inline tester.

On chainsaws and other really small engines, I like to use a set of jumper cables to help ensure the plug is securely grounded; bad ground will lead to a misdiagnosis.

Now let’s check it with an inline spark tester tool. We’ll check it in two ways, static and dynamic.

The smart thing about the inline tester – it allows us to run the saw with the tool in place (dynamic), that is great for intermittent spark issues.

One final test to run, this test eliminates a possible shorting issue with the On/Off switch and wiring; skipping this test could cause us to condemn the coil incorrectly.

Test as follows:

Two outcomes are possible:

Two – If you still have no spark after running these tests, and the coil air gap is good (see below), change out the coil; it’s failed. I’ve previously covered small engine coil replacement; you can check it out at the link below.

• Coil/armature replacement

3 Coil air gap adjustment – Lastly, we’ll check the air gap between the coil and flywheel is within spec. Typically, somewhere between 0.008″ – 0.012″. A shop may use a feeler gauge tool, but a business card (0.010″ approx.) works too.

I’ve covered the process in a little more detail here – Coil air-gap adjusting.

Engine Flooding

Engine flooding is commonly caused for a few reasons, top among them is incorrect hot starting. Now, I know you know how to start a hot saw, but it is worth covering for the folks who may be a little foggy on the difference between starting a hot saw and a cold one.

Typically, we don’t use a choke to start an already hot saw because doing so risks flooding the combustion chamber with excessive gas, flooding and fouling the spark plug.

Anyhow, here is the difference:

Flooding may also be caused by:

• Blocked air filter – clean air filter
• Stale gas – drain & clean gas tank
• Wrong gas type – drain & clean gas tank
• Too much oil mix – mix gas to correct ratio
• Dirty carburetor – clean carburetor
• Carburetor out of adjustment – adjust
• Carburetor gaskets worn out – replace gaskets
• Metering lever out of adjustment – adjust metering lever
• Faulty carburetor – replace carburetor
• Faulty plug – replace plug
• Bad spark plug gap – re-gap plug
• Faulty coil – replace coil

How to unflood chainsaw? There are three options:

I’d go with option 3.

Vacuum Leak

A vacuum leak refers to unmetered air entering the engine through unintended openings in the saw.

It’s the carburetor’s job to make sure this ratio is correct by adding the correct amount of gas for air that passes through its venturi. Even a small variation in the ratio hurts performance, and a large change in the ratio will cause a no-start.

Air that enters the engine unmetered will mean the engine is running with way less gas in the mix, a condition we call lean. Lean can be bad for a few reasons: it causes a saw to scream, run hot, and can cause engine damage; it can also cause a no-start condition.

Why does it only affect the saw when hot? Typically, for two reasons, a cold saw is started with full choke, meaning the mixture is rich with gas, and this helps counteract the lean condition.

The other usual reason is that the leak is only present as the saw gets hot. It’s normal for engine components to expand as they heat; gaskets and seals expand with the components to help seal these gaps. As gaskets and seals age, they are less capable of sealing.

Typical leaks include:

Other vacuum leaks are common, too, such as a worn-out carburetor and leaking engine impulse line.

How to diagnose? Diagnosis isn’t always easy; shop manuals will have you stripping components from the engine and using gauges to pressure and vacuum test for leaks. Tedious work, even with the correct tools. But we have a hack for you!

Next, idle the saw and try spraying the brake cleaner around the carburetor manifold and behind the clutch; a stall or change in engine speed indicates the affected area.

Cylinder Compression Issue

I’ve left a cylinder compression issue until last, not because it’s rare, but sorry to say it’s not. But mainly because it’s the hardest to diagnose – I don’t want to waste your time.

Proper combustion depends on the cylinder containing the pressure; a pressure loss means the air isn’t sufficiently heated or charged for ignition.

But wouldn’t a compression issue cause a no-start when the saw is cold too? In some cases, no. Testing compression such as a saw cold will read normal, but test when hot, and you’ll get a dramatic fall off.

As the engine heats up, metal expansion causes the rings to stick in the piston grooves, which isn’t great for compression. And as you’ve guessed, as the saw cools, the rings are released once again.

What’s a good reading? Compression varies by maker; best to check your particular model specs before testing, typically over 150 PSI.

If you don’t have a compression test kit, remove the muffler fasteners and remove the muffler. This allows us a clear window onto the hottest side of the piston – the exhaust side.

Turn over the engine slowly; finding scoring (heavy grooves and heat marks) on the piston indicates a compression issue. In addition, the engine often feels stiff to pull over even with the plug out, chain brake off, chain, and bar removed.

How to fix? In some cases, replacing the piston and rings will solve the problem, and such a mission is doable for the DIYer; that said, some small hobby saws are so inexpensive it may not be economical; a larger saw is a different story.

Sum Up

A chainsaw that won’t start when hot may be caused by a vapor lock, ignition system issues (such as faulty spark plugs or coils), engine flooding, vacuum leaks, or compression problems.

Vapor lock occurs when fuel evaporates and disrupts fuel flow. Ignition system faults can lead to weak or no spark. Engine flooding may result from incorrect hot starting. Vacuum leaks allow unmetered air into the engine, affecting the air-fuel ratio. Compression issues can cause pressure loss and difficulty in starting.